Magnetic recording medium having characterized magnetic layer and magnetic recording and reproducing device

ABSTRACT

The magnetic recording medium includes a non-magnetic support; and a magnetic layer including a ferromagnetic powder, in which an isoelectric point of a surface zeta potential of the magnetic layer after pressing the magnetic layer at a pressure of 70 atm is equal to or greater than 5.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese PatentApplication No. 2019-168025 filed on Sep. 17, 2019. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium and amagnetic recording and reproducing device.

2. Description of the Related Art

In recent years, magnetic recording media have been widely used asrecording media for data storage for recording and storing variouspieces of data (see, for example, JP2019-008849A).

SUMMARY OF THE INVENTION

Data recorded on a magnetic recording medium is generally reproduced byreading data recorded on a magnetic layer by sliding a magnetic head incontact with a surface of the magnetic layer while running the magneticrecording medium in a magnetic recording and reproducing device.However, in a case where the magnetic recording medium has deterioratedrunning stability, a reproducing output may decrease due to off-track.Thus, it is desirable that the magnetic recording medium has runningstability.

Data recorded on various recording media such as a magnetic recordingmedium is called hot data, warm data, and cold data depending on accessfrequencies (reproducing frequencies). The access frequencies decreasein the order of hot data, warm data, and cold data, and the recordingand storing of the data with low access frequency (for example, colddata) for a long period of time is referred to as “archive”. The dataamount recorded and stored on a recording medium for the archiveincreases in accordance with a dramatic increase in information contentsand digitization of various information in recent years, andaccordingly, a recording and reproducing system suitable for the archiveis gaining attention.

A magnetic recording medium capable of exhibiting excellent runningstability in a case of reproducing data after long-term storage asdescribed above is suitable as a recording medium for archiving.

In view of the above, one aspect of the invention provides for amagnetic recording medium having excellent running stability afterlong-term storage.

According to one aspect of the invention, there is provided a magneticrecording medium comprising: a non-magnetic support; and a magneticlayer including a ferromagnetic powder, in which an isoelectric point ofa surface zeta potential of the magnetic layer after pressing themagnetic layer at a pressure of 70 atm (hereinafter, also referred to asan “isoelectric point of a surface zeta potential of the magnetic layerafter pressing” or an “isoelectric point after pressing”) is equal to orgreater than 5.5.1 atm=101,325 Pa (Pascal)=101,325 N (Newton)/m².

In an embodiment, the isoelectric point may be 5.5 to 7.0.

In an embodiment, the magnetic layer may include inorganic oxide-basedparticles.

In an embodiment, the inorganic oxide-based particles may be compositeparticles of an inorganic oxide and a polymer.

In an embodiment, the magnetic layer may include a binding agent havingan acidic group.

In an embodiment, the acidic group may be at least one kind of acidicgroup selected from the group consisting of sulfonic acid groups andsalts thereof.

In an embodiment, the magnetic recording medium may further include anon-magnetic layer including a non-magnetic powder between thenon-magnetic support and the magnetic layer.

In an embodiment, the magnetic recording medium may include a backcoating layer including a non-magnetic powder on a surface of thenon-magnetic support opposite to the surface provided with the magneticlayer.

In an embodiment, the magnetic recording medium may be a magnetic tape.

According to another aspect of the invention, there is provided amagnetic recording and reproducing device comprising the magneticrecording medium; and a magnetic head.

According to one aspect of the invention, it is possible to provide amagnetic recording medium having excellent running stability afterlong-term storage. In addition, according to one aspect of theinvention, it is possible to provide a magnetic recording andreproducing device including the magnetic recording medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic Recording Medium

One embodiment of the invention relates to a magnetic recording mediumincluding a non-magnetic support and a magnetic layer including aferromagnetic powder, in which an isoelectric point of a surface zetapotential of the magnetic layer after pressing the magnetic layer at apressure of 70 atm is equal to or greater than 5.5.

Isoelectric Point of Surface Zeta Potential of Magnetic Layer afterPressing

The pressure of 70 atm for pressing the magnetic layer is a surfacepressure applied to a surface of the magnetic layer by pressing. Bycausing the magnetic recording medium to pass between two rolls whilecausing the magnetic recording medium to run at a speed of 20 m/min, thesurface pressure of 70 atm is applied to the surface of the magneticlayer. A tension of 0.5 N/m is applied to the running magnetic recordingmedium in a running direction. For example, for a tape-shaped magneticrecording medium (that is, a magnetic tape), a tension of 0.5 N/m isapplied in the longitudinal direction of the running magnetic tape. Thepressing is performed by causing the magnetic recording medium to passbetween two rolls six times in total and applying the surface pressureof 70 atm at each time when passing each roll. A metal roll is used asthe roll, and the roll is not heated. An environment for performing thepressing is an environment in which an atmosphere temperature is 20° C.to 25° C. and relative humidity is 40% to 60%. The magnetic recordingmedium to which the pressing is performed, is a magnetic recordingmedium which is not subjected to the long-term storage for 10 years orlonger in a room temperature environment of relative humidity of 40% to60%, and the storage corresponding to such long-term storage or anacceleration test corresponding to such long-term storage. The sameapplies to various physical properties relating to the magneticrecording medium described in the invention and the specification,unless otherwise noted.

The pressing described above can be performed by using a calendertreatment device used for manufacturing a magnetic recording medium. Forexample, a magnetic tape accommodated in a magnetic tape cartridge istaken out and caused to pass through calender rolls in the calendertreatment device, and accordingly, the magnetic tape can be pressed at apressure of 70 atm.

The inventors of the invention have conducted intensive studies toprovide a magnetic recording medium having excellent running stabilityafter long-term storage, and found that it is suitable to press themagnetic layer at a pressure of 70 atm in an acceleration testcorresponding to an example of archiving. This point will be furtherdescribed below.

For example, the magnetic tape is generally accommodated in a magnetictape cartridge in a state of being wound around a reel. Accordingly, thelong-term storage of the magnetic tape after the data with a low accessfrequency is recorded, is also performed in a state of beingaccommodated in the magnetic tape cartridge. In the magnetic tape woundaround a reel, a surface of a magnetic layer and a surface of a backcoating layer (in a case of including a back coating layer) or a surfaceof the non-magnetic support on a side opposite to a surface of themagnetic layer (in a case of not including a back coating layer) comeinto contact with each other, and accordingly, the magnetic layer ispressed in the magnetic tape cartridge. As a result of varioussimulation performed by the inventors, it is determined that it issuitable to press the magnetic layer at a pressure of 70 atm in theacceleration test corresponding to long-term storage (example ofarchive) for approximately 10 years in an environment of the atmospheretemperature of 20° C. to 25° C. and relative humidity of 40% to 60%(example of storage environment in archive). Therefore, the inventorsconducted a running stability test after pressing the magnetic layer at70 atm, and after intensive studies based on the results of this test,it is determined that the magnetic recording medium having anisoelectric point of equal to or greater than 5.5 after pressing hasexcellent running stability after pressing the magnetic layer at 70 atm,that is, running stability in a state corresponding to the abovelong-term storage. This point is a new finding that has not beenpreviously known and is not disclosed in JP2019-008849A.

A method for measuring the isoelectric point after pressing will bedescribed below. In the invention and the specification, the “surface ofthe magnetic layer” is identical to the surface of the magneticrecording medium on the magnetic layer side.

In the invention and the specification, the isoelectric point of thesurface zeta potential of the magnetic layer is a value of pH, in a casewhere a surface zeta potential measured by a flow potential method (alsoreferred to as a flow current method) becomes zero. A sample is cut outfrom the magnetic recording medium which is a measurement target, andthe sample is disposed in a measurement cell so that the surface of themagnetic layer comes into contact with an electrolyte. Pressure in themeasurement cell is changed to flow the electrolyte and a flow potentialat each pressure is measured, and then, the surface zeta potential isobtained by the following calculation expression.

$\zeta = {\frac{dI}{dp} \times \frac{\eta}{{ɛɛ}_{0}}\frac{L}{A}}$[ζ: surface zeta potential, p: pressure, I: flow potential, η: viscosityof electrolyte, ε: relative dielectric constant of electrolyte, ε₀:dielectric constant in a vacuum state, L: length of channel (flow pathbetween two electrodes), A: area of cross section of channel]

The pressure is changed in a range of 0 to 400,000 Pa (0 to 400 mbar).The calculation of the surface zeta potential by flowing the electrolyteto the measurement cell and measuring a flow potential is performed byusing electrolytes having different pH (from pH of 9 to pH of 3 atinterval of approximately 0.5). A total number of measurement points is13 from the measurement point of pH 9 to the 13th measurement points ofpH 3. By doing so, the surface zeta potentials of each measurement pointof pH is are obtained. As pH decreases, the surface zeta potentialdecreases. Thus, two measurement points at which polarity of the surfacezeta potential changes (a change from a positive value to a negativevalue) may appear, while pH decreases from 9 to 3. In a case where suchtwo measurement points appear, pH, in a case where the surface zetapotential is zero, is obtained by interpolation by using a straight line(linear function) showing a relationship between the surface zetapotential and pH of each of the two measurement points. Meanwhile, in acase where all of the surface zeta potentials obtained during thedecrease of pH from 9 to 3 is positive value, pH, in a case where thesurface zeta potential is zero, is obtained by extrapolation by using astraight line (linear function) showing a relationship between thesurface zeta potential and pH of the 13th measurement point (pH of 3)which is the final measurement point and the 12th measurement point. Onthe other hand, in a case where all of the surface zeta potentialsobtained during the decrease of pH from 9 to 3 is negative value, pH, ina case where the surface zeta potential is zero, is obtained byextrapolation by using a straight line (linear function) showing arelationship between the surface zeta potential and pH of the firstmeasurement point (pH of 9) which is the initial measurement point andthe 12th measurement point. By doing so, the value of pH, in a casewhere the surface zeta potential of the magnetic layer measured by theflow potential method is zero, is obtained.

The above measurement is performed three times in total at roomtemperature by using different samples cut out from the magneticrecording medium after the pressing, and pH, in a case where the surfacezeta potential of each measurement is zero, is obtained. For theviscosity and the relative dielectric constant of the electrolyte, ameasurement value at room temperature is used. In the invention and thespecification, the “room temperature” is in a range of 20° C. to 27° C.In regard to the magnetic layer, an arithmetical mean of three pHsobtained as described above is an isoelectric point of the surface zetapotential of the magnetic layer after the pressing of the magneticrecording medium which is a measurement target. As the electrolytehaving pH of 9, an electrolyte obtained by adjusting pH of a KCl aqueoussolution having a concentration of 1 mmol/L to 9 by using a KOH aqueoussolution having a concentration of 0.1 mol/L is used. As the electrolytehaving other pH, an electrolyte obtained by adjusting pH of theelectrolyte having pH of 9, which is adjusted as described above, byusing an HCl aqueous solution having a concentration of 0.1 mol/L isused. The isoelectric point of the surface zeta potential measured bythe above method is the isoelectric point obtained regarding the surfaceof the magnetic layer after the pressing.

The isoelectric point of the surface zeta potential of the magneticlayer after pressing of the magnetic recording medium is equal to orgreater than 5.5, from a viewpoint of improving running stability afterlong-term storage. In a case of reproducing data recorded on themagnetic recording medium after long-term storage, in a case where thecontact state between the surface of the magnetic layer and the magnetichead becomes unstable, the running stability may be deteriorated. Incontrast, in the magnetic recording medium in which the isoelectricpoint of the surface zeta potential of the magnetic layer after pressing(that is, isoelectric point of the surface zeta potential of themagnetic layer placed in a state corresponding to long-term storage) isequal to or greater than 5.5, that is, in a neutral to basic pH region,it is assumed that, a electrochemical reaction which hardly occursbetween the surface of the magnetic layer and the magnetic head and/orscraps generated due to chipping of the surface of the magnetic layerdue to contact between the surface of the magnetic layer and themagnetic head, which is hardly attached to the magnetic head contributeto stabilization of the contact state between the surface of themagnetic layer and the magnetic head. However, the invention is notlimited to the above surmise and other surmises described in thisspecification.

From a viewpoint of further improving running stability after long-termstorage, the isoelectric point of the surface zeta potential of themagnetic layer after pressing of the magnetic recording medium ispreferably equal to or greater than 5.7 and more preferably equal to orgreater than 6.0. As will be described later in detail, the isoelectricpoint of the surface zeta potential of the magnetic layer can becontrolled by the kind of a component used for forming the magneticlayer, a formation step of the magnetic layer, and the like. From aviewpoint of ease of control, the isoelectric point of the surface zetapotential of the magnetic layer is preferably equal to or less than 7.0,more preferably equal to or less than 6.7, and even more preferablyequal to or less than 6.5.

Hereinafter, the magnetic recording medium will be further described indetail.

Magnetic Layer

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer, a well-knownferromagnetic powder used as one kind or in combination of two or morekinds can be used as the ferromagnetic powder used in the magnetic layerof various magnetic recording media. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density. From this viewpoint, anaverage particle size of the ferromagnetic powder is preferably 50 nm orless, more preferably 45 nm or less, even more preferably 40 nm or less,further more preferably 35 nm or less, and still preferably 30 nm orless, still more preferably 25 nm or less, and still even morepreferably 20 nm or less. On the other hand, from a viewpoint ofmagnetization stability, the average particle size of the ferromagneticpowder is preferably 5 nm or more, more preferably 8 nm or more, evenmore preferably 10 nm or more, still preferably 15 nm or more, and stillmore preferably 20 nm.

Hexagonal Ferrite Powder

As a preferred specific example of the ferromagnetic powder, hexagonalferrite powder can be used. For details of the hexagonal ferrite powder,descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A,paragraphs 0134 to 0136 of JP2011-216149A, paragraphs 0013 to 0030 ofJP2012-204726A, and paragraphs 0029 to 0084 of JP2015-127985A can bereferred to, for example.

In the invention and the specification, the “hexagonal ferrite powder”is a ferromagnetic powder in which a hexagonal ferrite type crystalstructure is detected as a main phase by X-ray diffraction analysis. Themain phase is a structure to which a diffraction peak at the highestintensity in an X-ray diffraction spectrum obtained by the X-raydiffraction analysis belongs. For example, in a case where thediffraction peak at the highest intensity in the X-ray diffractionspectrum obtained by the X-ray diffraction analysis belongs to ahexagonal ferrite type crystal structure, it is determined that thehexagonal ferrite type crystal structure is detected as a main phase. Ina case where only a single structure is detected by the X-raydiffraction analysis, this detected structure is set as a main phase.The hexagonal ferrite type crystal structure includes at least an ironatom, a divalent metal atom, and an oxygen atom as constituting atoms. Adivalent metal atom is a metal atom which can be divalent cations asions, and examples thereof include an alkali earth metal atom such as astrontium atom, a barium atom, or a calcium atom, and a lead atom. Inthe invention and the specification, the hexagonal strontium ferritepowder is powder in which a main divalent metal atom included in thispowder is a strontium atom, and the hexagonal barium ferrite powder is apowder in which a main divalent metal atom included in this powder is abarium atom. The main divalent metal atom is a divalent metal atomoccupying the greatest content in the divalent metal atom included inthe powder based on atom %. However, the divalent metal atom describedabove does not include rare earth atom. The “rare earth atom” of theinvention and the specification is selected from the group consisting ofa scandium atom (Sc), an yttrium atom (Y), and a lanthanoid atom. Thelanthanoid atom is selected from the group consisting of a lanthanumatom (La), a cerium atom (Ce), a praseodymium atom (Pr), a neodymiumatom (Nd), a promethium atom (Pm), a samarium atom (Sm), an europiumatom (Eu), a gadolinium atom (Gd), a terbium atom (Tb), a dysprosiumatom (Dy), a holmium atom (Ho), an erbium atom (Er), a thulium atom(Tm), an ytterbium atom (Yb), and a lutetium atom (Lu).

Hereinafter, the hexagonal strontium ferrite powder which is oneembodiment of the hexagonal ferrite powder will be described morespecifically.

An activation volume of the hexagonal strontium ferrite powder ispreferably 800 to 1600 nm³. The atomized hexagonal strontium ferritepowder showing the activation volume in the range described above issuitable for manufacturing a magnetic recording medium exhibitingexcellent electromagnetic conversion characteristics. The activationvolume of the hexagonal strontium ferrite powder is preferably equal toor greater than 800 nm³, and can also be, for example, equal to orgreater than 850 nm³. In addition, from a viewpoint of further improvingthe electromagnetic conversion characteristics, the activation volume ofthe hexagonal strontium ferrite powder is more preferably equal to orsmaller than 1500 nm³, even more preferably equal to or smaller than1400 nm³, still preferably equal to or smaller than 1300 nm³, still morepreferably equal to or smaller than 1200 nm³, and still even morepreferably equal to or smaller than 1100 nm³. The same applies to theactivation volume of the hexagonal barium ferrite powder.

The “activation volume” is a unit of magnetization reversal and an indexshowing a magnetic magnitude of the particles. Regarding the activationvolume and an anisotropy constant Ku which will be described laterdisclosed in the invention and the specification, magnetic field sweeprates of a coercivity Hc measurement part at time points of 3 minutesand 30 minutes are measured by using an oscillation sample typemagnetic-flux meter (measurement temperature: 23° C.×1° C.), and theactivation volume and the anisotropy constant Ku are values acquiredfrom the following relational expression of Hc and an activation volumeV. A unit of the anisotropy constant Ku is 1 erg/cc=1.0×10⁻¹ J/m³.Hc=2Ku/Ms{1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant (unit: J/m³), Ms: saturationmagnetization (unit: kA/m), k: Boltzmann's constant, T: absolutetemperature (unit: K), V: activation volume (unit: cm³), A: spinprecession frequency (unit: s⁻¹), and t: magnetic field reversal time(unit: s)]

The anisotropy constant Ku can be used as an index of reduction ofthermal fluctuation, that is, improvement of thermal stability. Thehexagonal strontium ferrite powder can preferably have Ku equal to orgreater than 1.8×105 J/m³, and more preferably have Ku equal to orgreater than 2.0×105 J/m³. In addition, Ku of the hexagonal strontiumferrite powder can be, for example, equal to or smaller than 2.5×105J/m³. However, the high Ku is preferable, because it means high thermalstability, and thus, Ku is not limited to the exemplified value.

The hexagonal strontium ferrite powder may or may not include the rareearth atom. In a case where the hexagonal strontium ferrite powderincludes the rare earth atom, a content (bulk content) of the rare earthatom is preferably 0.5 to 5.0 atom % with respect to 100 atom % of theiron atom. In one embodiment, the hexagonal strontium ferrite powderincluding the rare earth atom can have rare earth atom surface layerportion uneven distribution. The “rare earth atom surface layer portionuneven distribution” of the invention and the specification means that acontent of rare earth atom with respect to 100 atom % of iron atom in asolution obtained by partially dissolving the hexagonal strontiumferrite powder with acid (hereinafter, referred to as a “rare earth atomsurface layer portion content” or simply a “surface layer portioncontent” regarding the rare earth atom) and a content of rare earth atomwith respect to 100 atom % of iron atom in a solution obtained bytotally dissolving the hexagonal strontium ferrite powder with acid(hereinafter, referred to as a “rare earth atom bulk content” or simplya “bulk content” regarding the rare earth atom) satisfy a ratio of rareearth atom surface layer portion content/rare earth atom bulk content>1.0.

The content of rare earth atom of the hexagonal strontium ferrite powderwhich will be described later is identical to the rare earth atom bulkcontent. With respect to this, the partial dissolving using acid is todissolve the surface layer portion of particles configuring thehexagonal strontium ferrite powder, and accordingly, the content of rareearth atom in the solution obtained by the partial dissolving is thecontent of rare earth atom in the surface layer portion of the particlesconfiguring the hexagonal strontium ferrite powder. The rare earth atomsurface layer portion content satisfying a ratio of “rare earth atomsurface layer portion content/rare earth atom bulk content >1.0” meansthat the rare earth atoms are unevenly distributed in the surface layerportion (that is, a larger amount of the rare earth atoms is present,compared to that inside), among the particles configuring the hexagonalstrontium ferrite powder. The surface layer portion of the invention andthe specification means a part of the region of the particlesconfiguring the hexagonal strontium ferrite powder from the inside fromthe surface.

In a case where the hexagonal strontium ferrite powder includes the rareearth atom, a content (bulk content) of the rare earth atom ispreferably 0.5 to 5.0 atom % with respect to 100 atom % of the ironatom. It is thought that the hexagonal strontium ferrite powderincluding the rare earth atom having the bulk content in the rangedescribed above and uneven distribution of the rare earth atom in thesurface layer portion of the particles configuring the hexagonalstrontium ferrite powder contribute to the prevention of reduction ofreproduction output during the repeated reproduction. It is surmisedthat this is because the anisotropy constant Ku can be increased due tothe hexagonal strontium ferrite powder including the rare earth atomhaving the bulk content in the range described above and unevendistribution of the rare earth atom in the surface layer portion of theparticles configuring the hexagonal strontium ferrite powder. As thevalue of the anisotropy constant Ku is high, occurrence of a phenomenon,so-called thermal fluctuation can be prevented (that is, thermalstability can be improved). By preventing the occurrence of thermalfluctuation, it is possible to prevent reduction of the reproductionoutput during the repeated reproduction. It is surmised that, the unevendistribution of the rare earth atom in the particle surface layerportion of the hexagonal strontium ferrite powder contributes tostabilization of a spin at an iron (Fe) site in a crystal lattice of thesurface layer portion, thereby increasing the anisotropy constant Ku.

It is surmised that the use of the hexagonal strontium ferrite powderhaving the rare earth atom surface layer portion uneven distribution asthe ferromagnetic powder of the magnetic layer contributes to theprevention of chipping of the surface of the magnetic layer due to thesliding with the magnetic head. That is, it is surmised that thehexagonal strontium ferrite powder having the rare earth atom surfacelayer portion uneven distribution also contributes to the improvement ofrunning durability of the magnetic recording medium. It is surmised thatthis is because the uneven distribution of the rare earth atom on thesurface of the particles configuring the hexagonal strontium ferritepowder contributes to improvement of an interaction between the surfaceof the particles and an organic substance (for example, binding agentand/or additive) included in the magnetic layer, thereby improvinghardness of the magnetic layer.

From a viewpoint of further preventing reduction of the reproductionoutput in the repeated reproduction and/or a viewpoint of furtherimproving running durability, the content of rare earth atom (bulkcontent) is more preferably 0.5 to 4.5 atom %, even more preferably 1.0to 4.5 atom %, and still preferably 1.5 to 4.5 atom %.

The bulk content is a content obtained by totally dissolving thehexagonal strontium ferrite powder. In the invention and thespecification, the content of the atom is a bulk content obtained bytotally dissolving the hexagonal strontium ferrite powder, unlessotherwise noted. The hexagonal strontium ferrite powder including therare earth atom may include only one kind of rare earth atom or mayinclude two or more kinds of rare earth atom, as the rare earth atom. Ina case where two or more kinds of rare earth atom are included, the bulkcontent is obtained from the total of the two or more kinds of rareearth atom. The same also applies to the other components of theinvention and the specification. That is, for a given component, onlyone kind may be used or two or more kinds may be used, unless otherwisenoted. In a case where two or more kinds are used, the content is acontent of the total of the two or more kinds.

In a case where the hexagonal strontium ferrite powder includes the rareearth atom, the rare earth atom included therein may be any one or morekinds of the rare earth atom. Examples of the rare earth atom preferablefrom a viewpoint of further preventing reduction of the reproductionoutput during the repeated reproduction include a neodymium atom, asamarium atom, an yttrium atom, and a dysprosium atom, a neodymium atom,a samarium atom, an yttrium atom are more preferable, and a neodymiumatom is even more preferable.

In the hexagonal strontium ferrite powder having the rare earth atomsurface layer portion uneven distribution, a degree of unevendistribution of the rare earth atom is not limited, as long as the rareearth atom is unevenly distributed in the surface layer portion of theparticles configuring the hexagonal strontium ferrite powder. Forexample, regarding the hexagonal strontium ferrite powder having therare earth atom surface layer portion uneven distribution, a ratio ofthe surface layer portion content of the rare earth atom obtained bypartial dissolving performed under the dissolving conditions which willbe described later and the bulk content of the rare earth atom obtainedby total dissolving performed under the dissolving conditions which willbe described later, “surface layer portion content/bulk content” isgreater than 1.0 and can be equal to or greater than 1.5. The “surfacelayer portion content/bulk content” greater than 1.0 means that the rareearth atom is unevenly distributed in the surface layer portion (thatis, a larger amount of the rare earth atoms is present, compared to thatinside), among the particles configuring the hexagonal strontium ferritepowder. In addition, the ratio of the surface layer portion content ofthe rare earth atom obtained by partial dissolving performed under thedissolving conditions which will be described later and the bulk contentof the rare earth atom obtained by total dissolving performed under thedissolving conditions which will be described later, “surface layerportion content/bulk content” can be, for example, equal to or smallerthan 10.0, equal to or smaller than 9.0, equal to or smaller than 8.0,equal to or smaller than 7.0, equal to or smaller than 6.0, equal to orsmaller than 5.0, or equal to or smaller than 4.0. However, in thehexagonal strontium ferrite powder having the rare earth atom surfacelayer portion uneven distribution, the “surface layer portioncontent/bulk content” is not limited to the exemplified upper limit orthe lower limit, as long as the rare earth atom is unevenly distributedin the surface layer portion of the particles configuring the hexagonalstrontium ferrite powder.

The partial dissolving and the total dissolving of the hexagonalstrontium ferrite powder will be described below. Regarding thehexagonal strontium ferrite powder present as the powder, sample powderfor the partial dissolving and the total dissolving are collected frompowder of the same lot. Meanwhile, regarding the hexagonal strontiumferrite powder included in a magnetic layer of a magnetic recordingmedium, a part of the hexagonal strontium ferrite powder extracted fromthe magnetic layer is subjected to the partial dissolving and the otherpart is subjected to the total dissolving. The extraction of thehexagonal strontium ferrite powder from the magnetic layer can beperformed by a method disclosed in a paragraph 0032 of JP2015-091747A.

The partial dissolving means dissolving performed so that the hexagonalstrontium ferrite powder remaining in the solution can be visuallyconfirmed at the time of the completion of the dissolving. For example,by performing the partial dissolving, a region of the particlesconfiguring the hexagonal strontium ferrite powder which is 10% to 20%by mass with respect to 100% by mass of a total of the particles can bedissolved. On the other hand, the total dissolving means dissolvingperformed until the hexagonal strontium ferrite powder remaining in thesolution is not visually confirmed at the time of the completion of thedissolving.

The partial dissolving and the measurement of the surface layer portioncontent are, for example, performed by the following method. However,dissolving conditions such as the amount of sample powder and the likedescribed below are merely examples, and dissolving conditions capableof performing the partial dissolving and the total dissolving can berandomly used.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 1 mol/L is held on ahot plate at a set temperature of 70° C. for 1 hour. The obtainedsolution is filtered with a membrane filter having a hole diameter of0.1 μm. The element analysis of the filtrate obtained as described aboveis performed by an inductively coupled plasma (ICP) analysis device. Bydoing so, the rare earth atom surface layer portion content with respectto 100 atom % of the iron atom can be obtained. In a case where aplurality of kinds of rare earth atoms are detected from the elementanalysis, a total content of the entirety of the rare earth atoms is thesurface layer portion content. The same applies to the measurement ofthe bulk content.

Meanwhile, the total dissolving and the measurement of the bulk contentare, for example, performed by the following method.

A vessel (for example, beaker) containing 12 mg of sample powder and 10ml of hydrochloric acid having a concentration of 4 mol/L is held on ahot plate at a set temperature of 80° C. for 3 hours. After that, theprocess is performed in the same manner as in the partial dissolving andthe measurement of the surface layer portion content, and the bulkcontent with respect to 100 atom % of the iron atom can be obtained.

From a viewpoint of increasing reproducing output in a case ofreproducing data recorded on a magnetic recording medium, it isdesirable that the mass magnetization σs of ferromagnetic powderincluded in the magnetic recording medium is high. In regards to thispoint, in hexagonal strontium ferrite powder which includes the rareearth atom but does not have the rare earth atom surface layer portionuneven distribution, σs tends to significantly decrease, compared tothat in hexagonal strontium ferrite powder not including the rare earthatom. With respect to this, it is thought that the hexagonal strontiumferrite powder having the rare earth atom surface layer portion unevendistribution is preferable for preventing such a significant decrease inσs. In one embodiment, σs of the hexagonal strontium ferrite powder canbe equal to or greater than 45 A·m²/kg and can also be equal to orgreater than 47 A·m²/kg. On the other hand, from a viewpoint of noisereduction, σs is preferably equal to or smaller than 80 A·m²/kg and morepreferably equal to or smaller than 60 A·m²/kg. σs can be measured byusing a well-known measurement device capable of measuring magneticproperties such as an oscillation sample type magnetic-flux meter. Inthe invention and the specification, the mass magnetization σs is avalue measured at a magnetic field strength of 15 kOe, unless otherwisenoted. 1 [kOe]=(10⁶/4π) [A/m]

Regarding the content (bulk content) of the constituting atom in thehexagonal strontium ferrite powder, a content of the strontium atom canbe, for example, 2.0 to 15.0 atom % with respect to 100 atom % of theiron atom. In one embodiment, in the hexagonal strontium ferrite powder,the divalent metal atom included in this powder can be only a strontiumatom. In another embodiment, the hexagonal strontium ferrite powder canalso include one or more kinds of other divalent metal atoms, inaddition to the strontium atom. For example, a barium atom and/or acalcium atom can be included. In a case where the other divalent metalatom other than the strontium atom is included, a content of a bariumatom and a content of a calcium atom in the hexagonal strontium ferritepowder respectively can be, for example, 0.05 to 5.0 atom % with respectto 100 atom % of the iron atom.

As the crystal structure of the hexagonal ferrite, a magnetoplumbitetype (also referred to as an “M type”), a W type, a Y type, and a Z typeare known. The hexagonal strontium ferrite powder may have any crystalstructure. The crystal structure can be confirmed by X-ray diffractionanalysis. In the hexagonal strontium ferrite powder, a single crystalstructure or two or more kinds of crystal structure can be detected bythe X-ray diffraction analysis. For example, in one embodiment, in thehexagonal strontium ferrite powder, only the M type crystal structurecan be detected by the X-ray diffraction analysis. For example, the Mtype hexagonal ferrite is represented by a compositional formula ofAFe₁₂O₁₉. Here, A represents a divalent metal atom, in a case where thehexagonal strontium ferrite powder has the M type, A is only a strontiumatom (Sr), or in a case where a plurality of divalent metal atoms areincluded as A, the strontium atom (Sr) occupies the hexagonal strontiumferrite powder with the greatest content based on atom % as describedabove. A content of the divalent metal atom in the hexagonal strontiumferrite powder is generally determined according to the type of thecrystal structure of the hexagonal ferrite and is not particularlylimited. The same applies to a content of an iron atom and a content ofan oxygen atom. The hexagonal strontium ferrite powder at least includesan iron atom, a strontium atom, and an oxygen atom, and can furtherinclude a rare earth atom. In addition, the hexagonal strontium ferritepowder may or may not include atoms other than these atoms. As anexample the hexagonal strontium ferrite powder may include an aluminumatom (Al). A content of the aluminum atom can be, for example, 0.5 to10.0 atom % with respect to 100 atom % of the iron atom. From aviewpoint of further preventing the reduction of the reproducing outputduring the repeated reproduction, the hexagonal strontium ferrite powderincludes the iron atom, the strontium atom, the oxygen atom, and therare earth atom, and a content of the atoms other than these atoms ispreferably equal to or smaller than 10.0 atom %, more preferably 0 to5.0 atom %, and may be 0 atom % with respect to 100 atom % of the ironatom. That is, in one embodiment, the hexagonal strontium ferrite powdermay not include atoms other than the iron atom, the strontium atom, theoxygen atom, and the rare earth atom. The content shown with atom %described above is obtained by converting a value of the content (unit:% by mass) of each atom obtained by totally dissolving the hexagonalstrontium ferrite powder into a value shown as atom % by using theatomic weight of each atom. In addition, in the invention and thespecification, a given atom which is “not included” means that thecontent thereof obtained by performing total dissolving and measurementby using an ICP analysis device is 0% by mass. A detection limit of theICP analysis device is generally equal to or smaller than 0.01 ppm(parts per million) based on mass. The expression “not included” is usedas a meaning including that a given atom is included with the amountsmaller than the detection limit of the ICP analysis device. In oneembodiment, the hexagonal strontium ferrite powder does not include abismuth atom (Bi).

Metal Powder

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. For details of theferromagnetic metal powder, descriptions disclosed in paragraphs 0137 to0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A canbe referred to, for example.

ε-Iron Oxide Powder

As a preferred specific example of the ferromagnetic powder, an ε-ironoxide powder can also be used. In the invention and the specification,the “ε-iron oxide powder” is a ferromagnetic powder in which an ε-ironoxide type crystal structure is detected as a main phase by X-raydiffraction analysis. For example, in a case where the diffraction peakat the highest intensity in the X-ray diffraction spectrum obtained bythe X-ray diffraction analysis belongs to an ε-iron oxide type crystalstructure, it is determined that the ε-iron oxide type crystal structureis detected as a main phase. As a producing method of the ε-iron oxidepowder, a producing method from a goethite, and a reverse micelle methodare known. All of the producing methods is well known. For example, fora method of producing the ε-iron oxide powder in which a part of Fe issubstituted with a substitutional atom such as Ga, Co, Ti, Al, or Rh, adescription disclosed in J. Jpn. Soc. Powder Metallurgy Vol. 61Supplement, No. S1, pp. S280-5284, J. Mater. Chem. C, 2013, 1, pp.5200-5206 can be referred to, for example. However, the producing methodof the ε-iron oxide powder which can be used as the ferromagnetic powderin the magnetic layer of the magnetic recording medium is not limited tothe method described here.

The activation volume of the ε-iron oxide powder is preferably in arange of 300 to 1500 nm³. The atomized ε-iron oxide powder showing theactivation volume in the range described above is suitable formanufacturing a magnetic recording medium exhibiting excellentelectromagnetic conversion characteristics. The activation volume of theε-iron oxide powder is preferably equal to or greater than 300 nm³, andcan also be, for example, equal to or greater than 500 nm³. In addition,from a viewpoint of further improving the electromagnetic conversioncharacteristics, the activation volume of the ε-iron oxide powder ismore preferably equal to or smaller than 1400 nm³, even more preferablyequal to or smaller than 1300 nm³, still preferably equal to or smallerthan 1200 nm³, and still more preferably equal to or smaller than 1100nm³.

The anisotropy constant Ku can be used as an index of reduction ofthermal fluctuation, that is, improvement of thermal stability. Theε-iron oxide powder can preferably have Ku equal to or greater than3.0×10⁴ J/m³, and more preferably have Ku equal to or greater than8.0×10⁴ J/m³. In addition, Ku of the ε-iron oxide powder can be, forexample, equal to or smaller than 3.0×10⁵ J/m³. However, the high Ku ispreferable, because it means high thermal stability, and thus, Ku is notlimited to the exemplified value.

From a viewpoint of increasing reproducing output in a case ofreproducing data recorded on a magnetic recording medium, it isdesirable that the mass magnetization σs of ferromagnetic powderincluded in the magnetic recording medium is high. In regard to thispoint, in one embodiment, σs of the ε-iron oxide powder can be equal toor greater than 8 A·m²/kg and can also be equal to or greater than 12A·m²/kg. On the other hand, from a viewpoint of noise reduction, σs ofthe ε-iron oxide powder is preferably equal to or smaller than 40A·m²/kg and more preferably equal to or smaller than 35 A·m²/kg.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at an imaging magnification ratio of 100,000 with atransmission electron microscope, the image is printed on photographicprinting paper so that the total magnification ratio of 500,000 toobtain an image of particles configuring the powder. A target particleis selected from the obtained image of particles, an outline of theparticle is traced with a digitizer, and a size of the particle (primaryparticle) is measured. The primary particle is an independent particlewhich is not aggregated.

The measurement described above is performed regarding 500 particlesrandomly extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of a plurality ofparticles is not limited to an embodiment in which particles configuringthe aggregate directly come into contact with each other, but alsoincludes an embodiment in which a binding agent, an additive, or thelike which will be described later is interposed between the particles.A term, particles may be used for representing the powder.

As a method of collecting a sample powder from the magnetic recordingmedium in order to measure the particle size, a method disclosed in aparagraph 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted,

(1) in a case where the shape of the particle observed in the particleimage described above is a needle shape, a fusiform shape, or a columnarshape (here, a height is greater than a maximum long diameter of abottom surface), the size (particle size) of the particles configuringthe powder is shown as a length of a long axis configuring the particle,that is, a long axis length,

(2) in a case where the shape of the particle is a planar shape or acolumnar shape (here, a thickness or a height is smaller than a maximumlong diameter of a plate surface or a bottom surface), the particle sizeis shown as a maximum long diameter of the plate surface or the bottomsurface, and

(3) in a case where the shape of the particle is a sphere shape, apolyhedron shape, or an unspecified shape, and the long axis configuringthe particles cannot be specified from the shape, the particle size isshown as an equivalent circle diameter. The equivalent circle diameteris a value obtained by a circle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter. In a case of the definition (3), the averageparticle size is an average diameter (also referred to as an averageparticle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50% to 90% by mass and more preferably 60%to 90% by mass. A high filling percentage of the ferromagnetic powder inthe magnetic layer is preferable from a viewpoint of improvement ofrecording density.

Binding Agent

The magnetic recording medium can be a coating type magnetic recordingmedium, and can include a binding agent in the magnetic layer. Thebinding agent is one or more kinds of resin. As the binding agent,various resins generally used as the binding agent of the coating typemagnetic recording medium can be used. For example, as the bindingagent, a resin selected from a polyurethane resin, a polyester resin, apolyamide resin, a vinyl chloride resin, an acrylic resin obtained bycopolymerizing styrene, acrylonitrile, or methyl methacrylate, acellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin,and a polyvinylalkylal resin such as polyvinyl acetal or polyvinylbutyral can be used alone or a plurality of resins can be mixed witheach other to be used. Among these, a polyurethane resin, an acrylicresin, a cellulose resin, and a vinyl chloride resin are preferable. Theresin may be a homopolymer or a copolymer. These resins can be used asthe binding agent even in the non-magnetic layer and/or a back coatinglayer which will be described later.

For the binding agent described above, description disclosed inparagraphs 0028 to 0031 of JP2010-024113A can be referred to. An averagemolecular weight of the resin used as the binding agent can be, forexample, 10,000 to 200,000 as a weight-average molecular weight. Theweight-average molecular weight of the invention and the specificationis a value obtained by performing polystyrene conversion of a valuemeasured by gel permeation chromatography (GPC) under the followingmeasurement conditions. The weight-average molecular weight of thebinding agent shown in examples which will be described later is a valueobtained by performing polystyrene conversion of a value measured underthe following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)′ 30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one embodiment, as the binding agent, a binding agent including anacidic group can be used. The “acidic group” of the invention and thespecification is used as a meaning including a state of a group capableof emitting H⁺ in water or a solvent including water (aqueous solvent)to dissociate anions and salt thereof. Specific examples of the acidicgroup include a sulfonic acid group, a sulfuric acid group, a carboxygroup, a phosphoric acid group, and salt thereof. For example, salt ofsulfonic acid group (—SO₃H) is represented by —SO₃M, and M represents agroup representing an atom (for example, alkali metal atom or the like)which may be cations in water or in an aqueous solvent. The same appliesto embodiments of salt of various groups described above. As an exampleof the binding agent including the acidic group, a resin including atleast one kind of acidic group selected from the group consisting of asulfonic acid group and salt thereof (for example, a polyurethane resinor a vinyl chloride resin) can be used. However, the resin included inthe magnetic layer is not limited to these resins. In addition, in thebinding agent including the acidic group, a content of the acidic groupcan be, for example, 0.03 to 0.50 meq/g. “eq” indicates equivalent andis a unit not convertible into SI unit. The content of variousfunctional groups such as the acidic group included in the resin can beobtained by a well-known method in accordance with the kind of thefunctional group. The amount of the binding agent used in a magneticlayer forming composition can be, for example, 1.0 to 30.0 parts by masswith respect to 100.0 parts by mass of the ferromagnetic powder.

In regards to the controlling of the isoelectric point of the surfacezeta potential of the magnetic layer, it is surmised that formation ofthe magnetic layer so that the amount of the acidic component present ina surface layer portion of the magnetic layer decreases contributes to adecrease in value of the isoelectric point. In addition, it is surmisedthat an increase in the amount of a basic component present in thesurface layer portion of the magnetic layer also contributes to anincrease in value of the isoelectric point. However, even in a casewhere such a magnetic layer is formed, in a case where the acidiccomponent moves to the surface layer portion from the inside of themagnetic layer, and/or the basic component moves from the surface layerportion of the magnetic layer to the inside thereof, by pressing thesurface of the magnetic layer during the long-term storage, the value ofthe isoelectric point of the surface zeta potential of the magneticlayer may be decreased after the long-term storage. In contrast, theisoelectric point of the surface zeta potential of the magnetic layerafter the pressing (that is, the isoelectric point of the surface zetapotential of the magnetic layer placed in a state corresponding to thelong-term storage) can be equal to or greater than 5.5, for example, byusing a projection formation agent which will be described later. Theinventors have surmised that this can improve running stability afterlong-term storage. The acidic component is used as a meaning including astate of a component capable of emitting H⁺ in water or an aqueoussolvent to dissociate anions and salt thereof. The basic component isused as a meaning including a state of a component capable of emittingOH⁻ in water or an aqueous solvent to dissociate cations and saltthereof. For example, in a case of using the acidic component, it isthought that performing the process of decreasing the amount of theacidic component in the surface layer portion after performing theprocess of unevenly distributing the acidic component to the surfacelayer portion of the coating layer of the magnetic layer formingcomposition contributes to an increase in the value of the isoelectricpoint of the surface zeta potential of the magnetic layer to control itto be equal to or greater than 5.5. For example, it is thought that, ina step of applying a magnetic layer forming composition onto anon-magnetic support directly or through a non-magnetic layer, theapplying which is performed in an alternating magnetic field by applyingan alternating magnetic field contributes to the uneven distribution ofthe acidic component to the surface layer portion of the coating layerof the magnetic layer forming composition. In addition, it is surmisedthat a burnishing process performed subsequent thereto contributes toremoval of at least some acidic component unevenly distributed. Theburnishing process is a process of rubbing a surface of a process targetwith a member (for example, abrasive tape or a grinding tool such as ablade for grinding or a wheel for grinding). A magnetic layer formingstep including the burnishing process will be described later in detail.As the acidic component, for example, a binding agent including anacidic group can be used.

In addition, a curing agent can also be used together with the resinwhich can be used as the binding agent. As the curing agent, in oneembodiment, a thermosetting compound which is a compound in which acuring reaction (crosslinking reaction) proceeds due to heating can beused, and in another embodiment, a photocurable compound in which acuring reaction (crosslinking reaction) proceeds due to lightirradiation can be used. At least a part of the curing agent is includedin the magnetic layer in a state of being reacted (crosslinked) withother components such as the binding agent, by proceeding the curingreaction in the magnetic layer forming step. This point is the same asregarding a layer formed by using a composition, in a case where thecomposition used for forming the other layer includes the curing agent.The preferred curing agent is a thermosetting compound, polyisocyanateis suitable. For details of the polyisocyanate, descriptions disclosedin paragraphs 0124 and 0125 of JP2011-216149A can be referred to, forexample. The amount of the curing agent can be, for example, 0 to 80.0parts by mass with respect to 100.0 parts by mass of the binding agentin the magnetic layer forming composition, and is preferably 50.0 to80.0 parts by mass, from a viewpoint of improvement of hardness of themagnetic layer.

Additives

The magnetic layer may include one or more kinds of additives, asnecessary, together with the various components described above. As theadditives, a commercially available product can be suitably selected andused according to the desired properties. Alternatively, a compoundsynthesized by a well-known method can be used as the additives. As theadditives, the curing agent described above is used as an example. Inaddition, examples of the additive included in the magnetic layerinclude a non-magnetic filler, a lubricant, a dispersing agent, adispersing assistant, an antibacterial agent, an antistatic agent, andan antioxidant. The non-magnetic filler is identical to the non-magneticparticles or non-magnetic powder. As the non-magnetic filler, anon-magnetic filler which can function as a projection formation agentand a non-magnetic filler which can function as an abrasive can be used.As the additive, a well-known additive such as various polymersdisclosed in paragraphs 0030 to 0080 of JP2016-051493A can also be used.

Projection Formation Agent

As the projection formation agent which is one embodiment of thenon-magnetic filler, particles of an inorganic substance can be used,particles of an organic substance can be used, and composite particlesof the inorganic substance and the organic substance can also be used.Examples of the inorganic substance include inorganic oxide such asmetal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide, and inorganic oxide is preferable. In oneembodiment, the projection formation agent can be inorganic oxide-basedparticles. Here, “-based” means “-containing”. One embodiment of theinorganic oxide-based particles is particles consisting of inorganicoxide. Another embodiment of the inorganic oxide-based particles iscomposite particles of inorganic oxide and an organic substance, and asa specific example, composite particles of inorganic oxide and a polymercan be used. As such particles, for example, particles obtained bybinding a polymer to a surface of the inorganic oxide particle can beused.

An average particle size of the projection formation agent is, forexample, 30 to 300 nm and is preferably 40 to 200 nm. As the shape ofthe particles is a shape close to a sphere, indentation resistanceexerted during a large pressure is applied is small, and accordingly,the particles are easily pushed into the magnetic layer. With respect tothis, in a case where the shape of the particles is a shape other thanthe sphere, for example, a shape of a so-called deformed shape, a largeindentation resistance is easily exerted, in a case where a largepressure is applied, and accordingly, particles are hardly pushed intothe magnetic layer. In addition, regarding the particles having a lowsurface smoothness in which a surface of the particle is not even, theindentation resistance is easily exerted, in a case where a largepressure is applied, and accordingly, the particles are hardly pushedinto the magnetic layer. It is thought that, in a case where theparticles which are easily pushed into the magnetic layer are includedin the magnetic layer, the basic component may move to the inside fromthe surface layer portion of the magnetic layer and/or the acidiccomponent may move from the inside of the magnetic layer to the surfacelayer portion, due to the particles pushed into the magnetic layer dueto pressure. On the other hand, it is surmised that, in a case where theparticles of the projection formation agent are hardly pushed into themagnetic layer by pressing, it is possible to prevent the basiccomponent from moving inside from the surface layer portion of themagnetic layer and/or the acidic component from moving from the insideof the magnetic layer to the surface layer portion. That is, it issurmised that the use of the projection formation agent hardly pushedinto the magnetic layer by pressing contributes to controlling theisoelectric point of the surface zeta potential of the magnetic layerafter pressing to be equal to or greater than 5.5.

Abrasive

The abrasive which is another embodiment of the non-magnetic filler ispreferably a non-magnetic powder having Mohs hardness exceeding 8 andmore preferably a non-magnetic powder having Mohs hardness equal to orgreater than 9. With respect to this, the Mohs hardness of theprojection formation agent can be, for example, equal to or smaller than8 or equal to or smaller than 7. A maximum value of Mohs hardness is 10of diamond. Specific examples thereof include powders of alumina(Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂, TiC, chromium oxide(Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), iron oxide, diamond, andthe like, and among these, alumina powder such as α-alumina and siliconcarbide powder are preferable. An average particle size of the abrasiveis, for example, in a range of 30 to 300 nm and preferably in a range of50 to 200 nm.

From a viewpoint of causing the projection formation agent and theabrasive to exhibit these functions in more excellent manner, a contentof the projection formation agent in the magnetic layer is preferably1.0 to 4.0 parts by mass and more preferably 1.2 to 3.5 parts by mass,with respect to 100.0 parts by mass of the ferromagnetic powder.Meanwhile, a content of the abrasive in the magnetic layer is preferably1.0 to 20.0 parts by mass, more preferably 3.0 to 15.0 parts by mass,and even more preferably 4.0 to 10.0 parts by mass, with respect to100.0 parts by mass of the ferromagnetic powder.

As an example of the additive which can be used in the magnetic layerincluding the abrasive, a dispersing agent disclosed in paragraphs 0012to 0022 of JP2013-131285A can be used as a dispersing agent forimproving dispersibility of the abrasive in the magnetic layer formingcomposition. In addition, for the dispersing agent, a descriptiondisclosed in paragraphs 0061 and 0071 of JP2012-133837A can be referredto. The dispersing agent may be included in the non-magnetic layer. Forthe dispersing agent which may be included in the non-magnetic layer, adescription disclosed in a paragraph 0061 of JP2012-133837A can bereferred to. In addition, for example, for the lubricant, a descriptiondisclosed in paragraphs 0030 to 0033, 0035, and 0036 of JP2016-126817Acan be referred to. The non-magnetic layer may include the lubricant.For the lubricant which may be included in the non-magnetic layer, adescription disclosed in paragraphs 0030, 0031, 0034, 0035, and 0036 ofJP2016-126817A can be referred to.

The magnetic layer described above can be provided on the surface of thenon-magnetic support directly or indirectly through the non-magneticlayer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic recordingmedium may include a magnetic layer directly on the non-magnetic supportor may include a non-magnetic layer including the non-magnetic powderbetween the non-magnetic support and the magnetic layer. Thenon-magnetic powder used in the non-magnetic layer may be powder of aninorganic substance or powder of an organic substance. In addition,carbon black and the like can be used. Examples of the inorganicsubstance include metal, metal oxide, metal carbonate, metal sulfate,metal nitride, metal carbide, and metal sulfide. These non-magneticpowder can be purchased as a commercially available product or can bemanufactured by a well-known method. For details thereof, descriptionsdisclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referredto. For carbon black capable of being used in the non-magnetic layer, adescription disclosed in paragraphs 0040 and 0041 of JP2010-024113A canbe referred to. The content (filling percentage) of the non-magneticpowder of the non-magnetic layer is preferably 50% to 90% by mass andmore preferably 60% to 90% by mass.

The non-magnetic layer can include a binding agent and can also includeadditives. In regards to other details of a binding agent or additivesof the non-magnetic layer, the well-known technology regarding thenon-magnetic layer can be applied. In addition, in regards to the typeand the content of the binding agent, and the type and the content ofthe additive, for example, the well-known technology regarding themagnetic layer can be applied.

The non-magnetic layer of the invention and the specification alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 100 Oe, or alayer having a residual magnetic flux density equal to or smaller than10 mT and coercivity equal to or smaller than 100 Oe. It is preferablethat the non-magnetic layer does not have a residual magnetic fluxdensity and coercivity.

Non-Magnetic Support

Next, the non-magnetic support will be described. As the non-magneticsupport (hereinafter, also simply referred to as a “support”),well-known components such as polyethylene terephthalate, polyethylenenaphthalate, polyamide, polyamide imide, aromatic polyamide subjected tobiaxial stretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heat treatmentmay be performed with respect to these supports in advance.

Back Coating Layer

The magnetic recording medium may or may not include a back coatinglayer including a non-magnetic powder on a surface of the non-magneticsupport opposite to the surface provided with the magnetic layer. Theback coating layer preferably includes any one or both of carbon blackand inorganic powder. The back coating layer can include a binding agentand can also include additives. In regards to the binding agent includedin the back coating layer and various additives, a well-known technologyregarding the back coating layer can be applied, and a well-knowntechnology regarding the list of the magnetic layer and/or thenon-magnetic layer can also be applied. For example, for the backcoating layer, descriptions disclosed in paragraphs 0018 to 0020 ofJP2006-331625A and page 4, line 65, to page 5, line 38, of U.S. Pat. No.7,029,774B can be referred to.

Various Thicknesses

A thickness of the non-magnetic support is preferably 3.00 to 20.00 μm,more preferably 3.00 to 10.00 μm, and even more preferably 3.00 to 6.00μm.

A thickness of the magnetic layer can be optimized according to asaturation magnetization amount of a magnetic head used, a head gaplength, a recording signal band, and the like. The thickness of themagnetic layer is generally 0.01 μm to 0.15 μm, preferably 0.02 μm to0.12 μm, and more preferably 0.03 μm to 0.10 μm, from a viewpoint ofrealization of high-density recording. The magnetic layer may be atleast one layer, or the magnetic layer can be separated to two or morelayers having magnetic properties, and a configuration regarding awell-known multilayered magnetic layer can be applied. A thickness ofthe magnetic layer which is separated into two or more layers is a totalthickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and even more preferably 0.10 to 0.70 μm.

The thicknesses of various layers and the non-magnetic support of themagnetic recording medium can be obtained by a well-known film thicknessmeasurement method. As an example, a cross section of the magneticrecording medium in a thickness direction is exposed by a well-knownmethod of ion beams or microtome, and the exposed cross section isobserved with a scanning electron microscope. In the cross sectionobservation, various thicknesses can be obtained as the thicknessobtained at one portion, or as an arithmetical mean of thicknessesobtained at a plurality of portions which are two or more portionsrandomly extracted, for example, two portions. Alternatively, thethickness of each layer may be obtained as a designed thicknesscalculated under the manufacturing conditions.

Manufacturing Step

Preparation of Each Layer Forming Composition

Composition for forming the magnetic layer, the non-magnetic layer, orthe back coating layer generally include a solvent, together with thevarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. The amount of solvent in each layerforming composition is not particularly limited, and can be identical tothat in each layer forming composition of a typical coating typemagnetic recording medium. A step of preparing the composition forforming each layer can generally include at least a kneading step, adispersing step, and a mixing step provided before and after thesesteps, as necessary. Each step may be divided into two or more stages.Components used in the preparation of each layer forming composition maybe added at the beginning or during any step. In addition, eachcomponent may be separately added in two or more steps.

In order to prepare each layer forming composition, a well-knowntechnology can be used. In the kneading step, an open kneader, acontinuous kneader, a pressure kneader, or a kneader having a strongkneading force such as an extruder is preferably used. For details ofthe kneading processes, descriptions disclosed in JP1989-106338A(JP-H01-106338A) and JP1989-079274A (JP-H01-079274A) can be referred to.In addition, in order to disperse each layer forming composition, one ormore kinds of dispersion beads selected from the group consisting ofglass beads and other dispersion beads can be used as a dispersionmedium. As such dispersion beads, zirconia beads, titania beads, andsteel beads which are dispersion beads having high specific gravity aresuitable. These dispersion beads may be used by optimizing a particlediameter (bead diameter) and a filling percentage of the dispersionbeads. As a disperser, a well-known disperser can be used. Each layerforming composition may be filtered by a well-known method beforeperforming the coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a hole diameter of 0.01 to 3 μm (for example, filter made ofglass fiber or filter made of polypropylene) can be used, for example.

In one embodiment, in the step of preparing the magnetic layer formingcomposition, a dispersion liquid including a projection formation agent(hereinafter, referred to as a “projection formation agent liquid”) canbe prepared, and then this projection formation agent liquid can bemixed with one or more other components of the magnetic layer formingcomposition. For example, the projection formation agent liquid, adispersion liquid including an abrasive (hereinafter, referred to as an“abrasive solution”), and a dispersion liquid including a ferromagneticpowder (hereinafter, referred to as a “magnetic liquid”) are separatelyprepared, mixed, and dispersed, thereby preparing the magnetic layerforming composition. It is preferable to separately prepare variousdispersion liquids in order to improve the dispersibility of theferromagnetic powder, the projection formation agent, and the abrasivein the magnetic layer forming composition. For example, the projectionformation agent liquid can be prepared by a well-known dispersionprocess such as ultrasonic process. The ultrasonic treatment can beperformed for about 1 to 300 minutes at an ultrasonic output of about 10to 2,000 watts per 200 cc (1 cc=1 cm³). In addition, the filtering maybe performed after a dispersion process. For the filter used for thefiltering, the above description can be referred to.

The magnetic layer can be formed, for example, by directly applying themagnetic layer forming composition onto the surface of the non-magneticsupport or performing multilayer coating of the magnetic layer formingcomposition with the non-magnetic layer forming composition in order orat the same time. The back coating layer can be formed by applying theback coating layer forming composition to a surface of the non-magneticsupport opposite to a surface provided with the magnetic layer or to beprovided with the magnetic layer. For details of the coating for formingeach layer, a description disclosed in a paragraph 0066 ofJP2010-231843A can be referred to.

It is surmised that this is because, an acidic component (for example,the binding agent including an acidic group) is easily unevenlydistributed to a surface layer portion of a coating layer of themagnetic layer forming composition due to the application of themagnetic layer forming composition performed in the alternating magneticfield, and thus, by drying this coating layer, it is possible tounevenly distribute the acidic component to the surface layer portion ofthe magnetic layer. In addition, it is surmised that a burnishingprocess performed subsequent thereto contributes to removal of at leastsome acidic component unevenly distributed to control the isoelectricpoint of the surface zeta potential of the magnetic layer to be equal toor greater than 5.5.

The applying of the alternating magnetic field can be performed bydisposing a magnet in a coating device so that the alternating magneticfield is applied vertically to the surface of the coating layer of themagnetic layer forming composition. A magnetic field strength of thealternating magnetic field can be, for example, set as approximately0.05 to 3.00 T. However, there is no limitation to this range. The“vertical” in the invention and the specification does not mean only avertical direction in the strict sense, but also includes a range oferrors allowed in the technical field of the invention. For example, therange of errors means a range of less than ±10° from an exact verticaldirection.

The burnishing process is a process of rubbing a surface of a processtarget with a member (for example, abrasive tape or a grinding tool suchas a blade for grinding or a wheel for grinding) and can be performed inthe same manner as a well-known burnishing process for manufacturing acoating type magnetic recording medium. The burnishing process can bepreferably performed by performing one or both of rubbing (polishing) ofa surface of a coating layer which is a process target with an abrasivetape, and rubbing (grinding) of a surface of a coating layer which is aprocess target with a grinding tool. As the abrasive tape, acommercially available product may be used or an abrasive tapemanufactured by a well-known method may be used. In addition, as thegrinding tool, a well-known blade for grinding such as a fixed typeblade, a diamond wheel, or a rotary blade, or a wheel for grinding canbe used. Further, a wiping process of wiping the surface of the coatinglayer rubbed with the abrasive tape and/or the grinding tool with awiping material may be performed. For details of the preferable abrasivetape, grinding tool, burnishing process, and wiping process, paragraphs0034 to 0048, FIG. 1, and examples of JP1994-052544A (JP-H06-052544A)can be referred to. It is thought that, as the burnishing process isreinforced, it is possible remove a large amount of the acidic componentunevenly distributed to the surface layer portion of the coating layerof the magnetic layer forming composition by performing the applying inthe alternating magnetic field. As an abrasive having high hardness isused as an abrasive included in the abrasive tape, the burnishingprocess can be reinforced, and as the amount of the abrasive in theabrasive tape increases, the burnishing process can be reinforced. Inaddition, as a grinding tool having high hardness is used as thegrinding tool, the burnishing process can be reinforced. In regards toburnishing process conditions, as a sliding speed of the surface of thecoating layer which is the process target and the member (for example,the abrasive tape or the grinding tool) increases, the burnishingprocess can be reinforced. The sliding speed can be increased byincreasing one or both of a speed of movement of the member and a speedof movement of the magnetic tape of the process target. Although thereason is not clear, as the amount of the binding agent including theacidic group in the coating layer of the magnetic layer formingcomposition is great, the isoelectric point of a surface zeta potentialof the magnetic layer tends to increase after the burnishing process.

In a case where the magnetic layer forming composition includes a curingagent, a curing process is preferably performed in any stage of the stepfor forming the magnetic layer. The burnishing process is preferablyperformed at least before the curing process. After the curing process,the burnishing process may be further performed. The inventors havethought that it is preferable to perform the burnishing process beforethe curing process, in order to increase removal efficiency for removingthe acidic component from the surface layer portion of the coating layerof the magnetic layer forming composition. The curing process can beperformed by a process of a heat treatment or light irradiation,according to the kind of the curing agent included in the magnetic layerforming composition. The curing process conditions are not particularlylimited and may be suitably set according to the list of the magneticlayer forming composition, the kind of the curing agent, the thicknessof the coating layer, and the like. For example, in a case where thecoating layer is formed by using the magnetic layer forming compositionincluding polyisocyanate as the curing agent, the curing process ispreferably a heat treatment.

The surface smoothing treatment can be preferably performed before thecuring process. The surface smoothing treatment is a process performedfor increasing smoothness of the surface of the magnetic recordingmedium and is preferably performed by a calender process. For details ofthe calender process, description disclosed in a paragraph 0026 ofJP2010-231843A can be referred to, for example.

For various other steps for manufacturing the magnetic recording medium,a well-known technology can be applied. For details of the varioussteps, descriptions disclosed in paragraphs 0067 to 0070 ofJP2010-231843A can be referred to, for example. For example, it ispreferable that the coating layer of the magnetic layer formingcomposition is subjected to an alignment process, while the coatinglayer is wet (not dried). For the alignment process, various well-knowntechnologies such as descriptions disclosed in a paragraph 0067 ofJP2010-231843A can be used. For example, a homeotropic alignment processcan be performed by a well-known method such as a method using adifferent polar opposing magnet. In the alignment zone, a drying speedof the coating layer can be controlled by a temperature, an air flow ofthe dry air and/or a transporting rate of the magnetic tape in thealignment zone. In addition, the coating layer may be preliminarilydried before transporting to the alignment zone. In a case of performingthe alignment process, it is preferable to apply a magnetic field (forexample, DC magnetic field) for aligning the ferromagnetic powder withrespect to the coating layer of the magnetic layer forming compositionapplied in the alternating magnetic field.

A servo pattern can be formed on the magnetic recording mediummanufactured as described above by a well-known method, in order torealize tracking control of a magnetic head of the magnetic recordingand reproducing device and control of a running speed of the magneticrecording medium. The “formation of the servo pattern” can be “recordingof a servo signal”. In one embodiment, the magnetic recording medium canbe a tape-shaped magnetic recording medium (magnetic tape), and inanother embodiment, may be a disk-shaped magnetic recording medium(magnetic disc). Hereinafter, the formation of the servo pattern will bedescribed using a magnetic tape as an example.

The servo pattern is generally formed along a longitudinal direction ofthe magnetic tape. As a method of control using a servo signal (servocontrol), timing-based servo (TBS), amplitude servo, or frequency servois used.

As shown in European Computer Manufacturers Association (ECMA)-319, atiming-based servo system is used in a magnetic tape based on alinear-tape-open (LTO) standard (generally referred to as an “LTOtape”). In this timing-based servo system, the servo pattern isconfigured by continuously disposing a plurality of pairs of magneticstripes (also referred to as “servo stripes”) not parallel to each otherin a longitudinal direction of the magnetic tape. As described above, areason for that the servo pattern is configured with one pair ofmagnetic stripes not parallel to each other is because a servo signalreading element passing on the servo pattern recognizes a passageposition thereof. Specifically, one pair of the magnetic stripes areformed so that a gap thereof is continuously changed along the widthdirection of the magnetic tape, and a relative position of the servopattern and the servo signal reading element can be recognized, by thereading of the gap thereof by the servo signal reading element. Theinformation of this relative position can realize the tracking of a datatrack. Accordingly, a plurality of servo tracks are generally set on theservo pattern along the width direction of the magnetic tape.

The servo band is configured of a servo signal continuous in thelongitudinal direction of the magnetic tape. A plurality of servo bandsare normally provided on the magnetic tape. For example, the numberthereof is 5 in the LTO tape. A region interposed between two adjacentservo bands is called a data band. The data band is configured of aplurality of data tracks and each data track corresponds to each servotrack.

In one embodiment, as shown in JP2004-318983A, information showing thenumber of servo band (also referred to as “servo band identification(ID)” or “Unique Data Band Identification Method (UDIM) information”) isembedded in each servo band. This servo band ID is recorded by shiftinga specific servo stripe among the plurality of pair of servo stripes inthe servo band so that the position thereof is relatively deviated inthe longitudinal direction of the magnetic tape. Specifically, theposition of the shifted specific servo stripe among the plurality ofpair of servo stripes is changed for each servo band. Accordingly, therecorded servo band ID becomes unique for each servo band, andtherefore, the servo band can be uniquely specified by only reading oneservo band by the servo signal reading element.

In a method of uniquely specifying the servo band, a staggered method asshown in ECMA-319 is used. In this staggered method, the group of onepair of magnetic stripes (servo stripe) not parallel to each other whichare continuously disposed in the longitudinal direction of the magnetictape is recorded so as to be shifted in the longitudinal direction ofthe magnetic tape for each servo band. A combination of this shiftedservo band between the adjacent servo bands is set to be unique in theentire magnetic tape, and accordingly, the servo band can also beuniquely specified by reading of the servo pattern by two servo signalreading elements.

In addition, as shown in ECMA-319, information showing the position inthe longitudinal direction of the magnetic tape (also referred to as“Longitudinal Position (LPOS) information”) is normally embedded in eachservo band. This LPOS information is recorded so that the position ofone pair of servo stripes are shifted in the longitudinal direction ofthe magnetic tape, in the same manner as the UDIM information. However,unlike the UDIM information, the same signal is recorded on each servoband in this LPOS information.

Other information different from the UDIM information and the LPOSinformation can be embedded in the servo band. In this case, theembedded information may be different for each servo band as the UDIMinformation, or may be common in all of the servo bands, as the LPOSinformation.

In addition, as a method of embedding the information in the servo band,a method other than the method described above can be used. For example,a predetermined code may be recorded by thinning out a predeterminedpair among the group of pairs of the servo stripes.

A servo pattern forming head is also referred to as a servo write head.The servo write head includes pairs of gaps corresponding to the pairsof magnetic stripes by the number of servo bands. In general, a core anda coil are respectively connected to each of the pairs of gaps, and amagnetic field generated in the core can generate leakage magnetic fieldin the pairs of gaps, by supplying a current pulse to the coil. In acase of forming the servo pattern, by inputting a current pulse whilecausing the magnetic tape to run on the servo write head, the magneticpattern corresponding to the pair of gaps is transferred to the magnetictape, and the servo pattern can be formed. A width of each gap can besuitably set in accordance with a density of the servo patterns to beformed. The width of each gap can be set as, for example, equal to orsmaller than 1 μm, 1 to 10 μm, or equal to or greater than 10 μm.

Before forming the servo pattern on the magnetic tape, a demagnetization(erasing) process is generally performed on the magnetic tape. Thiserasing process can be performed by applying a uniform magnetic field tothe magnetic tape by using a DC magnet and an AC magnet. The erasingprocess includes direct current (DC) erasing and alternating current(AC) erasing. The AC erasing is performed by slowing decreasing anintensity of the magnetic field, while reversing a direction of themagnetic field applied to the magnetic tape. Meanwhile, the DC erasingis performed by adding the magnetic field in one direction to themagnetic tape. The DC erasing further includes two methods. A firstmethod is horizontal DC erasing of applying the magnetic field in onedirection along a longitudinal direction of the magnetic tape. A secondmethod is vertical DC erasing of applying the magnetic field in onedirection along a thickness direction of the magnetic tape. The erasingprocess may be performed with respect to all of the magnetic tape or maybe performed for each servo band of the magnetic tape.

A direction of the magnetic field to the servo pattern to be formed isdetermined in accordance with the direction of erasing. For example, ina case where the horizontal DC erasing is performed to the magnetictape, the formation of the servo pattern is performed so that thedirection of the magnetic field and the direction of erasing becomesopposite to each other. Accordingly, the output of the servo signalobtained by the reading of the servo pattern can be increased. Asdisclosed in JP2012-053940A, in a case where the magnetic pattern istransferred to the magnetic tape subjected to the vertical DC erasing byusing the gap, the servo signal obtained by the reading of the formedservo pattern has a unipolar pulse shape. Meanwhile, in a case where themagnetic pattern is transferred to the magnetic tape subjected to thehorizontal DC erasing by using the gap, the servo signal obtained by thereading of the formed servo pattern has a bipolar pulse shape.

The magnetic tape is normally accommodated in a magnetic tape cartridgeand the magnetic tape cartridge is mounted on a magnetic recording andreproducing device.

In the magnetic tape cartridge, the magnetic tape is generallyaccommodated in a cartridge main body in a state of being wound around areel. The reel is rotatably provided in the cartridge main body. As themagnetic tape cartridge, a single reel type magnetic tape cartridgeincluding one reel in a cartridge main body and a twin reel typemagnetic tape cartridge including two reels in a cartridge main body arewidely used. In a case where the single reel type magnetic tapecartridge is mounted in the magnetic recording and reproducing device inorder to record and/or reproduce data on the magnetic tape, the magnetictape is drawn from the magnetic tape cartridge and wound around the reelon the magnetic recording and reproducing device side. A magnetic headis disposed on a magnetic tape transportation path from the magnetictape cartridge to a winding reel. Sending and winding of the magnetictape are performed between a reel (supply reel) on the magnetic tapecartridge side and a reel (winding reel) on the magnetic recording andreproducing device side. In the meantime, the magnetic head comes intocontact with and slides on the surface of the magnetic layer of themagnetic tape, and accordingly, the recording and/or reproduction ofdata is performed. With respect to this, in the twin reel type magnetictape cartridge, both reels of the supply reel and the winding reel areprovided in the magnetic tape cartridge. The magnetic tape cartridge maybe any of single reel type magnetic tape cartridge and twin reel typemagnetic tape cartridge. A well-known technology can be applied forother details of the magnetic tape cartridge.

Magnetic Recording and Reproducing Device

According to another aspect of the invention, there is provided amagnetic recording and reproducing device comprising the magneticrecording medium; and a magnetic head.

In the invention and the specification, the “magnetic recording andreproducing device” means a device capable of performing at least one ofthe recording of data on the magnetic recording medium or thereproducing of data recorded on the magnetic recording medium. Such adevice is generally called a drive. The magnetic recording andreproducing device can be a sliding type magnetic recording andreproducing device. The sliding type magnetic recording and reproducingdevice is a device in which the surface of the magnetic layer and themagnetic head are in contact with each other and slide, in a case ofperforming recording of data on the magnetic recording medium and/orreproducing of the recorded data.

The magnetic head included in the magnetic recording and reproducingdevice can be a recording head capable of performing the recording ofdata on the magnetic recording medium, and can also be a reproducinghead capable of performing the reproducing of data recorded on themagnetic recording medium. In addition, in the embodiment, the magneticrecording and reproducing device can include both of a recording headand a reproducing head as separate magnetic heads. In anotherembodiment, the magnetic head included in the magnetic recording andreproducing device can also have a configuration of comprising both ofan element for recording data (recording element) and an element forreproducing data (reproducing element) in one magnetic head.Hereinafter, the element for recording data and the element forreproducing are collectively referred to as “elements for data”. As thereproducing head, a magnetic head (MR head) including a magnetoresistive(MR) element capable of reading data recorded on the magnetic tape withexcellent sensitivity as the reproducing element is preferable. As theMR head, various well-known MR heads such as an AnisotropicMagnetoresistive (AMR) head, a Giant Magnetoresistive (GMR) head, or aTunnel Magnetoresistive (TMR) can be used. In addition, the magnetichead which performs the recording of data and/or the reproducing of datamay include a servo signal reading element. Alternatively, as a headother than the magnetic head which performs the recording of data and/orthe reproducing of data, a magnetic head (servo head) including a servosignal reading element may be included in the magnetic recording andreproducing device. For example, the magnetic head which performs therecording of data and/or reproducing of the recorded data (hereinafter,also referred to as a “recording and reproducing head”) can include twoservo signal reading elements, and each of the two servo signal readingelements can read two adjacent servo bands at the same time. One or aplurality of elements for data can be disposed between the two servosignal reading elements.

In the magnetic recording and reproducing device, the recording of dataon the magnetic recording medium and/or the reproducing of data recordedon the magnetic recording medium can be performed by bringing thesurface of the magnetic layer of the magnetic recording medium intocontact with the magnetic head and sliding. The magnetic recording andreproducing device may include the magnetic recording medium accordingto the embodiment of the invention, and well-known technologies can beapplied for the other configurations.

For example, in a case of the recording and/or reproducing data withrespect to the magnetic recording medium in which the servo pattern isformed, first, the tracking is performed by using the servo signalobtained by reading the servo pattern. That is, as the servo signalreading element follows a predetermined servo track, the element fordata is controlled to pass on the target data track. The movement of thedata track is performed by changing the servo track to be read by theservo signal reading element in the tape width direction.

In addition, the recording and reproducing head can also perform therecording and/or reproducing with respect to other data bands. In thiscase, the servo signal reading element is moved to a predetermined servoband by using the UDIM information described above, and the trackingwith respect to the servo band may be started.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to embodiments shown in theexamples. “Parts” and “%” in the following description mean “parts bymass” and “% by mass”, unless otherwise noted. In addition, steps andevaluations described below are performed in an environment of anatmosphere temperature of 23° C.±1° C., unless otherwise noted.

Example 1

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo ChemicalIndustry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixedsolvent of methyl ethyl ketone and toluene) of a SO₃Na group-containingpolyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g)manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent ofmethyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solventwere mixed with 100.0 parts of alumina powder (HIT-80 manufactured bySumitomo Chemical Co., Ltd.) having a gelatinization ratio ofapproximately 65% and a Brunauer-Emmett-Teller (BET) specific surfacearea of 20 m²/g, and dispersed in the presence of zirconia beads by apaint shaker for 5 hours. After the dispersion, the dispersion liquidand the beads were separated by a mesh and an alumina dispersion wasobtained.

(2) List of Magnetic Layer Forming Composition

Magnetic Liquid

Ferromagnetic powder (Type: See Table 1) 100.0 parts

Binding agent (Type: See Table 1): see Table 1

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Solution

Alumina dispersion prepared in the section (1): 6.0 parts

Projection formation agent liquid

Projection formation agent (type: see Table 1): 1.3 parts

Methyl ethyl ketone: 9.0 parts

Cyclohexanone: 6.0 parts

Other Components

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Polyisocyanate (CORONATE (registered trademark) manufactured by TosohCorporation): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(3) List of Magnetic Layer Forming Composition

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

Average particle size (average long axis length): 0.15 μm

Average acicular ratio: 7

BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

Average particle size: 20 nm

Binding agent A: 18.0 parts

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

(4) List of Back Coating Layer Forming Composition

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

Average particle size (average long axis length): 0.15 μm

Average acicular ratio: 7

BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

Average particle size: 20 nm

Vinyl chloride copolymer: 13.0 parts

Sulfonate group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Methyl ethyl ketone: 155.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

(5) Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

A magnetic liquid was prepared by dispersing (beads-dispersing) variouscomponents with a batch type vertical sand mill for 24 hours. Asdispersion beads, zirconia beads having a bead diameter of 0.5 mm wereused.

The projection formation agent liquid was prepared by filtering adispersion liquid obtained by mixing the components of theabove-mentioned projection formation agent liquid and thenultrasonically treating (dispersing) for 60 minutes with an ultrasonicoutput of 500 watts per 200 cc by a horn-type ultrasonic dispersingdevice with a filter having a hole size of 0.5 μm.

Using the sand mill, the magnetic liquid, the abrasive solution, theprojection formation agent liquid, and other components (othercomponents and the finishing additive solvent) were mixed andbead-dispersed for 5 minutes, and a treatment (ultrasonic dispersion)was performed for 0.5 minutes using a batch type ultrasonic device (20kHz, 300 W). Thereafter, the mixture was filtered using a filter havinga hole size of 0.5 μm to prepare a magnetic layer forming composition.

The non-magnetic layer forming composition was prepared by the followingmethod.

Each component excluding the lubricant (stearic acid, stearic acidamide, and butyl stearate), cyclohexanone, and methyl ethyl ketone wasdispersed by using batch type vertical sand mill for 24 hours to obtaina dispersion liquid. As dispersion beads, zirconia beads having a beaddiameter of 0.5 mm were used. After that, the remaining components wereadded into the obtained dispersion liquid and stirred with a dissolver.The dispersion liquid obtained as described above was filtered with afilter having a hole diameter of 0.5 μm and a non-magnetic layer formingcomposition was prepared.

The back coating layer forming composition was prepared by the followingmethod.

Each component excluding polyisocyanate and cyclohexanone was kneaded byan open kneader and diluted, and was subjected to a dispersion processof 12 passes, with a transverse beads mill disperser and zirconia beadshaving a bead diameter of 1 mm, by setting a bead filling percentage as80 volume %, a circumferential speed of rotor distal end as 10 m/sec,and a retention time for 1 pass as 2 minutes. After that, the remainingcomponents were added into the obtained dispersion liquid and stirredwith a dissolver. The dispersion liquid obtained as described above wasfiltered with a filter having a hole diameter of 1 μm and a back coatinglayer forming composition was prepared.

(6) Manufacturing Method of Magnetic Tape

The non-magnetic layer forming composition prepared in the section (5)was applied to a surface of a biaxial stretching support made ofpolyethylene naphthalate having a thickness of 5.00 μm so that thethickness after the drying becomes 1.00 μm and was dried to form anon-magnetic layer.

Then, in a coating device disposed with a magnet for applying analternating magnetic field, the magnetic layer forming compositionprepared in the section (5) was applied onto the surface of thenon-magnetic layer so that the thickness after the drying becomes 0.10μm, while applying an alternating magnetic field (magnetic fieldstrength: 0.15 T), to form a coating layer. The applying of thealternating magnetic field was performed so that the alternatingmagnetic field was applied vertically to the surface of the coatinglayer. After that, a homeotropic alignment process was performed byapplying a direct current magnetic field having a magnetic fieldstrength of 0.30 T in a vertical direction with respect to a surface ofa coating layer, while the coating layer of the magnetic layer formingcomposition is wet (not dried). After that, the coating layer was driedto form a magnetic layer.

After that, the back coating layer forming composition prepared in thesection (5) was applied to the surface of the support made ofpolyethylene naphthalate on a side opposite to the surface where thenon-magnetic layer and the magnetic layer were formed, so that thethickness after the drying becomes 0.50 μm, and was dried to form a backcoating layer.

The magnetic tape obtained as described above was slit to have a widthof ½ inches (0.0127 meters), and the burnishing process and the wipingprocess of the surface of the coating layer of the magnetic layerforming composition were performed. The burnishing process and thewiping process were performed in a process device having a configurationshown in FIG. 1 of JP-H06-52544A, by using a commercially availableabrasive tape (product name: MA22000 manufactured by Fujifilm HoldingsCorporation, abrasive: diamond/Cr₂O₃/red oxide) as an abrasive tape, byusing a commercially available sapphire blade (manufactured by KyoceraCorporation, width of 5 mm, length of 35 mm, an angle of a distal end of60 degrees) as a blade for grinding, and by using a commerciallyavailable wiping material (product name: WRP736 manufactured by KurarayCo., Ltd.) as a wiping material. For the process conditions, processconditions of Example 12 of JP-H06-52544A were used.

After the burnishing process and the wiping process, a calender process(surface smoothing treatment) was performed by using a calender rollconfigured of only a metal roll, at a speed of 80 m/min, linear pressureof 294 kN/m (300 kg/cm), and a calender temperature (surface temperatureof a calender roll) of 100° C.

Then, the heat treatment (curing process) was performed in theenvironment of the atmosphere temperature of 70° C. for 36 hours tomanufacture a magnetic tape.

In a state where the magnetic layer of the manufactured magnetic tapewas demagnetized, servo patterns having disposition and shapes accordingto the LTO Ultrium format were formed on the magnetic layer by using aservo write head mounted on a servo writer. Accordingly, a magnetic tapeincluding data bands, servo bands, and guide bands in the dispositionaccording to the LTO Ultrium format in the magnetic layer, and includingservo patterns having the disposition and the shape according to the LTOUltrium format on the servo band was obtained.

Examples 2 to 21 and Comparative Examples 1 to 13

A magnetic tape was manufactured by the same method as in the Example 1,except that various conditions shown in Table 1 were changed as shown inTable 1.

In regard to the manufacturing method of the magnetic tape, as shown inTable 1, in Examples 2 to 21 and Comparative Examples 3 to 10, themanufacturing method of the magnetic tape same as in Example 1 wasperformed. That is, the application of the alternating magnetic fieldwas performed during coating of the magnetic layer forming compositionin the same manner as in Example 1, and the burnishing process and thewiping process were performed with respect to the coating layer of themagnetic layer forming composition.

In contrast, in Comparative Example 1, 2, 11 to 13, the manufacturingmethod of the magnetic tape same as in Example 1 was performed, exceptthat the burnishing process and the wiping process were not performedwith respect to the coating layer of the magnetic layer formingcomposition, without applying the alternating magnetic field during thecoating of the magnetic layer forming composition.

Projection Formation Agent

A projection formation agent used for manufacturing magnetic tape ofexamples or comparative examples is as follows. A projection formationagent 1 and a projection formation agent 3 are particles having a lowsurface smoothness of a surface of particles. A particle shape of aprojection formation agent 2 is a shape of a cocoon. A particle shape ofa projection formation agent 4 is a so-called indeterminate shape. Aparticle shape of a projection formation agent 5 is a shape closer to asphere.

Projection formation agent 1: ATLAS (composite particles of silica andpolymer) manufactured by Cabot Corporation, average particle size: 100nm

Projection formation agent 2: TGC6020N (silica particles) manufacturedby Cabot Corporation, average particle size: 140 nm

Projection formation agent 3: Cataloid (water dispersed sol of silicaparticles; as a projection formation agent for preparing a projectionformation agent liquid, a dried solid material obtained by removing thesolvent by heating the water dispersed sol described above is used)manufactured by JGC c&c, average particle size: 120 nm

Projection formation agent 4: ASAHI #50 (carbon black) manufactured byAsahi Carbon Co., Ltd., average particle size: 300 nm

Projection formation agent 5: PL-10L (water dispersed sol of silicaparticles; as a projection formation agent for preparing a projectionformation agent liquid, a dried solid material obtained by removing thesolvent by heating the water dispersed sol described above is used)manufactured by FUSO CHEMICAL CO., LTD., average particle size: 130 nm

Binding Agent

In Table 1, the “binding agent A” is a SO₃Na group-containingpolyurethane resin (weight-average molecular weight: 70,000, SO₃Nagroup: 0.20 meq/g).

In Table 1, the “binding agent B” is a vinyl chloride copolymer (productname: MR110, SO₃K group-containing vinyl chloride copolymer, SO₃K group:0.07 meq/g) manufactured by Kaneka Corporation.

Ferromagnetic Powder

In Table 1, “BaFe” is a hexagonal barium ferrite powder having anaverage particle size (average plate diameter) of 21 nm. “SrFe1” and“SrFe2” respectively indicate hexagonal strontium ferrite powder, and“ε-iron oxide” indicates ε-iron oxide powder.

The activation volume and the anisotropy constant Ku of the variousferromagnetic powders are values obtained by the method described aboveregarding each ferromagnetic powder by using an oscillation sample typemagnetic-flux meter (manufactured by Toei Industry Co., Ltd.).

The mass magnetization σs is a value measured using a oscillation sampletype magnetic-flux meter (manufactured by Toei Industry Co., Ltd.) at amagnetic field strength of 15 kOe.

Method 1 for Producing Hexagonal Strontium Ferrite Powder

In Table 1, “SrFe1” is a hexagonal strontium ferrite powder produced bythe following method.

1707 g of SrCO₃, 687 g of H3BO3, 1120 g of Fe2O3, 45 g of Al(OH)3, 24 gof BaCO3, 13 g of CaCO3, and 235 g of Nd2O3 were weighed and mixed in amixer to obtain a raw material mixture.

The obtained raw material mixture was dissolved in a platinum crucibleat a dissolving temperature of 1390° C., and a tap hole provided on thebottom of the platinum crucible was heated while stirring the dissolvedliquid, and the dissolved liquid was tapped in a rod shape atapproximately 6 g/sec. The tap liquid was rolled and cooled with a watercooling twin roller to prepare an amorphous body.

280 g of the prepared amorphous body was put into an electronic furnace,heated to 635° C. (crystallization temperature) at a rate of temperaturerise of 3.5° C./min, and held at the same temperature for 5 hours, andhexagonal strontium ferrite particles were precipitated (crystallized).

Then, the crystallized material obtained as described above includingthe hexagonal strontium ferrite particles was coarse-pulverized with amortar, 1000 g of zirconia beads having a particle diameter of 1 mm and800 ml of an acetic acid aqueous solution having a concentration of 1%were added to a glass bottle, and a dispersion process was performed ina paint shaker for 3 hours. After that, the obtained dispersion liquidand the beads were dispersed and put in a stainless still beaker. Thedispersion liquid was left at a liquid temperature of 100° C. for 3hours, subjected to a dissolving process of a glass component,precipitated with a centrifugal separator, decantation was repeated forcleaning, and drying was performed in a heating furnace at a furnaceinner temperature of 110° C. for 6 hours, to obtain hexagonal strontiumferrite powder.

Regarding the hexagonal strontium ferrite powder (in Table 1, “SrFe1”)obtained as described above, an average particle size was 18 nm, anactivation volume was 902 nm³, an anisotropy constant Ku was 2.2×105J/m³, and a mass magnetization σs was 49 A·m²/kg.

12 mg of a sample powder was collected from the hexagonal strontiumferrite powder obtained as described above, the element analysis of afiltrate obtained by the partial dissolving of this sample powder underthe dissolving conditions described above was performed by the ICPanalysis device, and a surface layer portion content of a neodymium atomwas obtained.

Separately, 12 mg of a sample powder was collected from the hexagonalstrontium ferrite powder obtained as described above, the elementanalysis of a filtrate obtained by the total dissolving of this samplepowder under the dissolving conditions described above was performed bythe ICP analysis device, and a bulk content of a neodymium atom wasobtained.

The content (bulk content) of the neodymium atom in the hexagonalstrontium ferrite powder obtained as described above with respect to 100atom % of iron atom was 2.9 atom %. In addition, the surface layerportion content of the neodymium atom was 8.0 atom %. A ratio of thesurface layer portion content and the bulk content, “surface layerportion content/bulk content” was 2.8 and it was confirmed that theneodymium atom is unevenly distributed on the surface layer of theparticles.

A crystal structure of the hexagonal ferrite shown by the powderobtained as described above was confirmed by scanning CuKα ray under thecondition of a voltage of 45 kV and intensity of 40 mA and measuring anX-ray diffraction pattern under the following conditions (X-raydiffraction analysis). The powder obtained as described above showed acrystal structure of magnetoplumbite type (M type) hexagonal ferrite. Inaddition, a crystal phase detected by the X-ray diffraction analysis wasa magnetoplumbite type single phase.

PANalytical X'Pert Pro diffractometer, PIXcel detector

Soller slit of incident beam and diffraction beam: 0.017 radians

Fixed angle of dispersion slit: ¼ degrees

Mask: 10 mm

Scattering prevention slit: ¼ degrees

Measurement mode: continuous

Measurement time per 1 stage: 3 seconds

Measurement speed: 0.017 degrees per second

Measurement step: 0.05 degree

Method 2 for Producing Hexagonal Strontium Ferrite Powder

In Table 1, “SrFe2” is a hexagonal strontium ferrite powder produced bythe following method.

1725 g of SrCO3, 666 g of H3BO3, 1332 g of Fe2O3, 52 g of Al(OH)3, 34 gof CaCO3, and 141 g of BaCO3 were weighed and mixed in a mixer to obtaina raw material mixture.

The obtained raw material mixture was dissolved in a platinum crucibleat a dissolving temperature of 1380° C., and a tap hole provided on thebottom of the platinum crucible was heated while stirring the dissolvedliquid, and the dissolved liquid was tapped in a rod shape atapproximately 6 g/sec. The tap liquid was rolled and cooled with a watercooling twin roll to prepare an amorphous body.

280 g of the obtained amorphous body was put into an electronic furnace,heated to 645° C. (crystallization temperature), and held at the sametemperature for 5 hours, and hexagonal strontium ferrite particles wereprecipitated (crystallized).

Then, the crystallized material obtained as described above includingthe hexagonal strontium ferrite particles was coarse-pulverized with amortar, 1000 g of zirconia beads having a particle diameter of 1 mm and800 ml of an acetic acid aqueous solution having a concentration of 1%were added to a glass bottle, and a dispersion process was performed ina paint shaker for 3 hours. After that, the obtained dispersion liquidand the beads were dispersed and put in a stainless still beaker. Thedispersion liquid was left at a liquid temperature of 100° C. for 3hours, subjected to a dissolving process of a glass component,precipitated with a centrifugal separator, decantation was repeated forcleaning, and drying was performed in a heating furnace at a furnaceinner temperature of 110° C. for 6 hours, to obtain hexagonal strontiumferrite powder.

Regarding the hexagonal strontium ferrite powder (in Table 1, “SrFe2”)obtained as described above, an average particle size was 19 nm, anactivation volume was 1102 nm3, an anisotropy constant Ku was 2.0×105J/m3, and a mass magnetization σs was 50 A·m2/kg.

Method for Producing ε-Iron Oxide Powder

In Table 1, “ε-iron oxide” is an ε-iron oxide powder produced by thefollowing method.

4.0 g of ammonia aqueous solution having a concentration of 25% wasadded to a material obtained by dissolving 8.3 g of iron (III) nitratenonahydrate, 1.3 g of gallium (III) nitrate octahydrate, 190 mg ofcobalt (II) nitrate hexahydrate, 150 mg of titanium (IV) sulfate, and1.5 g of polyvinyl pyrrolidone (PVP) in 90 g of pure water, whilestirring by using a magnetic stirrer, in an atmosphere under theconditions of an atmosphere temperature of 25° C., and the mixture wasstirred for 2 hours still under the temperature condition of theatmosphere temperature of 25° C. A citric acid aqueous solution obtainedby dissolving 1 g of citric acid in 9 g of pure water was added to theobtained solution and stirred for 1 hour. The powder precipitated afterthe stirring was collected by centrifugal separation, washed with purewater, and dried in a heating furnace at a furnace inner temperature of80° C.

800 g of pure water was added to the dried powder and the powder wasdispersed in water again, to obtain a dispersion liquid. The obtaineddispersion liquid was heated to a liquid temperature of 50° C., and 40 gof ammonia aqueous solution having a concentration of 25% was addeddropwise while stirring. The stirring was performed for 1 hour whileholding the temperature of 50° C., and 14 mL of tetraethoxysilane (TEOS)was added dropwise and stirred for 24 hours. 50 g of ammonium sulfatewas added to the obtained reaction solution, the precipitated powder wascollected by centrifugal separation, washed with pure water, and driedin a heating furnace at a furnace inner temperature of 80° C. for 24hours, and a precursor of ferromagnetic powder was obtained.

The heating furnace at a furnace inner temperature of 1000° C. wasfilled with the obtained precursor of ferromagnetic powder in theatmosphere and subjected to heat treatment for 4 hours.

The thermal-treated precursor of ferromagnetic powder was put intosodium hydroxide (NaOH) aqueous solution having a concentration of 4mol/L, the liquid temperature was held at 70° C., stirring was performedfor 24 hours, and accordingly, a silicon acid compound which was animpurity was removed from the thermal-treated precursor of ferromagneticpowder.

After that, by the centrifugal separation process, ferromagnetic powderobtained by removing the silicon acid compound was collected and washedwith pure water, and ferromagnetic powder was obtained.

The composition of the obtained ferromagnetic powder was confirmed byInductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), andGa, Co, and Ti substitution type ε-iron oxide(ε-Ga_(0.28)Co_(0.05)Ti_(0.05)Fe_(1.62)O₃) was obtained. In addition,the X-ray diffraction analysis was performed under the same conditionsas disclosed regarding the method 1 for producing the hexagonalstrontium ferrite powder described above, and it was confirmed that theobtained ferromagnetic powder has a crystal structure of a single phasewhich is an ε phase not including a crystal structure of an α phase anda γ phase (ε-iron oxide type crystal structure) from the peak of theX-ray diffraction pattern.

Regarding the obtained (ε-iron oxide powder (in Table 1, “ε-ironoxide”), an average particle size was 12 nm, an activation volume was746 nm3, an anisotropy constant Ku was 1.2×105 J/m3, and a massmagnetization σs was 16 A·m2/kg.

Evaluation Method

(1) Isoelectric Point of Surface Zeta Potential of Magnetic Layer afterPressing Each magnetic tape of the examples and the comparative exampleswas passed between two rolls (without heating the rolls) six times intotal while running the magnetic tape in a longitudinal direction at aspeed of 20 m/min in a state where a tension of 0.5 N/m was applied, byusing a calender treatment device including a 7-step calender rollconfigured of only a metal roll in an environment of an atmospheretemperature of 20° C. to 25° C. and relative humidity of 40% to 60%, andaccordingly, the pressing was performed by applying a surface pressureof 70 atm to the surface of each magnetic layer, during the passingbetween each roll.

Six samples for isoelectric point measurement were cut out from eachmagnetic tape of the examples and the comparative examples after thepressing and the two samples were disposed in the measurement cell inone measurement. In the measurement cell, a sample installing surfaceand a surface of the back coating layer of the sample were bonded toeach other by using a double-sided tape in upper and lower sample table(size of each sample installing surface is 1 cm×2 cm) of the measurementcell. By doing so, after disposing the two samples, in a case where anelectrolyte flows in the measurement cell, the surface of the magneticlayer of the two samples comes into contact with the electrolyte on theupper and lower sample table of the measurement cell, and thus, thesurface zeta potential of the surface of the magnetic layer can bemeasured. The measurement was performed three times in total by usingtwo samples in each measurement, and the isoelectric points of thesurface zeta potential of the magnetic layer were obtained. Anarithmetical mean of the obtained three values by the three times of themeasurements is shown in Table 1, as the isoelectric point of thesurface zeta potential of the magnetic layer of each magnetic tape afterpressing. As a surface zeta potential measurement device, SurPASSmanufactured by Anton Paar was used. The measurement conditions were setas follows. Other details of the method of obtaining the isoelectricpoint are as described above.

Measurement cell: variable gap cell (20 mm×10 mm)

Measurement mode: Streaming Current

Gap: approximately 200 μm

Measurement temperature: room temperature

Ramp Target Pressure/Time: 400,000 Pa (400 mbar)/60 seconds

Electrolyte: KCl aqueous solution having concentration of 1 mmol/L(adjusted pH to 9)

pH adjusting solution: HCl aqueous solution having concentration of 0.1mol/L or KOH aqueous solution having concentration of 0.1 mol/L

Measurement pH: pH 9→pH 3 (measured at 13 measurement points in total atinterval of approximately 0.5)

(2) Evaluation of Running Stability after Pressing at Pressure of 70 atm

Regarding each magnetic tape of the examples and the comparativeexamples, a position error signal (PES) was obtained by the followingmethod after the pressing in the section (1).

A servo pattern was read with a verifying head on a servo writer used inthe formation of the servo pattern. The verifying head is a magnetichead for reading for confirming quality of the servo pattern formed inthe magnetic tape, and an element for reading is disposed on a positioncorresponding to the position (position of the magnetic tape in a widthdirection) of the servo pattern, in the same manner as the magnetic headof the well-known magnetic tape device (drive).

In the verifying head, a well-known PES arithmetic circuit whichcalculates head positioning accuracy of the servo system as the PES isconnected from an electrical signal obtained by reading the servopattern in the verifying head. The PES arithmetic circuit calculates, asnecessary, displacement of the magnetic tape in a width direction fromthe input electrical signal (pulse signal), and a value obtained byapplying a high pass filter (cut-off: 500 cycles/m) with respect to atemporal change information (signal) of this displacement was calculatedas the PES. The PES can be an index of running stability and it ispossible to evaluate that the running stability is excellent, in a casewhere the PES calculated described above is equal to or smaller than 18nm.

The result described above is shown in Table 1 (Tables 1-1 to 1-5).

TABLE 1-1 Unit Example 1 Example 2 Example3 Example 4 Example 5 Example6 Example 7 Formation Ferromagnetic Type — BaFe BaFe BaFe BaFe BaFe BaFeBaFe of powder magnetic Projection Type — Projection ProjectionProjection Projection Projection Projection Projection layer formationformation formation formation formation formation formation formationagent agent 1 agent 2 agent 3 agent 1 agent 2 agent 3 agent 1 Content ofBinding Parts 5.0 5.0 5.0 20.0 20.0 20.0 0 binding agent agent A inmagnetic Binding Parts 0 0 0 0 0 0 10.0 liquid agent B Alternatingmagnetic field — Performed Performed Performed Performed PerformedPerformed Performed application during coating Burnishing process —Performed Performed Performed Performed Performed Performed PerformedIsoelectric point of surface zeta potential — 5.5 5.5 5.5 6.5 6.5 6.56.0 of magnetic layer after pressing PES nm 17 17 17 12 12 12 15

TABLE 1-2 Unit Example 8 Example 9 Example 10 Example 11 Example 12Example 13 Example 14 Formation Ferromagnetic Type — BaFe BaFe BaFe BaFeBaFe SrFe1 SrFe1 of powder magnetic Projection Type — ProjectionProjection Projection Projection Projection Projection Projection layerformation formation formation formation formation formation formationformation agent agent 2 agent 3 agent 1 agent 2 agent 3 agent 1 agent 2Content of Binding Parts 0 0 10.0 10.0 10.0 5.0 5.0 binding agent agentA in magnetic Binding Parts 10.0 10.0 10.0 10.0 10.0 0 0 liquid agent BAlternating magnetic field — Performed Performed Performed PerformedPerformed Performed Performed application during coating Burnishingprocess — Performed Performed Performed Performed Performed PerformedPerformed Isoelectric point of surface zeta potential — 6.0 6.0  6.4 6.4  6.4 5.5 5.5 of magnetic layer after pressing PES nm 15 15 17  17   17   17 17

TABLE 1-3 Unit Example 15 Example 16 Example 17 Example 18 Example 19Example 20 Example 21 Formation Ferromagnetic Type — SrFe1 SrFe2 SrFe2SrFe2 ε-iron oxide ε-iron oxide ε-iron oxide of powder magneticProjection Type — Projection Projection Projection Projection ProjectionProjection Projection layer formation formation formation formationformation formation formation formation agent agent 3 agent 1 agent 2agent 3 agent 1 agent 2 agent 3 Content of Binding Parts 5.0 5.0 5.0 5.05.0 5.0 5.0 binding agent agent A in magnetic Binding Parts 0 0 0 0 0 00 liquid agent B Alternating magnetic field — Performed PerformedPerformed Performed Performed Performed Performed application duringcoating Burnishing process — Performed Performed Performed PerformedPerformed Performed Performed Isoelectric point of surface zetapotential — 5.5 5.5 5.5 5.5 5.5 5.5 5.5 of magnetic layer after pressingPES nm 17 17 17 17 17 17 17

TABLE 1-4 Comparative Comparative Comparative Comparative ComparativeComparative Comparative Unit Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Example 7 Formation Ferromagnetic Type — BaFe BaFeBaFe BaFe BaFe BaFe BaFe of powder magnetic Projection Type — ProjectionProjection Projection Projection Projection Projection Projection layerformation formation formation formation formation formation formationformation agent agent 4 agent 5 agent 4 agent 5 agent 4 agent 5 agent 4Content of Binding Parts 5.0 5.0 5.0 5.0 20.0 20.0 0 binding agent agentA in magnetic Binding Parts 0 0 0 0 0 0 10.0 liquid agent B Alternatingmagnetic field — Not Not Performed Performed Performed PerformedPerformed application during coating performed performed Burnishingprocess — Not Not Performed Performed Performed Performed Performedperformed performed Isoelectric point of surface zeta potential — 5.05.0 5.3 5.3 5.3 5.3 5.2 of magnetic layer after pressing PES nm 30 30 2222 22 22 26

TABLE 1-5 Comparative Comparative Comparative Comparative ComparativeComparative Unit Example 8 Example 9 Example 10 Example 11 Example 12Example 13 Formation Ferromagnetic Type — BaFe BaFe BaFe BaFe BaFe BaFeof powder magnetic Projection Type — Projection Projection ProjectionProjection Projection Projection layer formation formation formationformation formation formation formation agent agent 5 agent 4 agent 5agent 1 agent 2 agent 3 Content of Binding Parts 0 10.0 10.0 5.0 5.0 5.0binding agent agent A in magnetic Binding Parts 10.0 10.0 10.0 0 0 0liquid agent B Alternating magnetic field — Performed PerformedPerformed Not Not Not application during coating performed performedperformed Burnishing process — Performed Performed Performed Not Not Notperformed performed performed Isoelectric point of surface zetapotential — 5.2  5.2  5.2 5.1 5.1 5.1 of magnetic layer after pressingPES nm 26 26   26   28 28 28

From the result shown in Table 1, it can be confirmed that, in all ofthe magnetic tapes of the examples, excellent running stability isexhibited after the pressing at a pressure of 70 atm, that is, in astate corresponding to the state after the long-term storage. Accordingto this magnetic tape, even after the magnetic tape is accommodated in astate of being wound around a reel for a long period of time in themagnetic tape cartridge, after information with a low access frequencyis recorded, the stable running can be performed in the magneticrecording and reproducing device, and the magnetic tape is suitable as arecording medium for archive.

One aspect of the invention is effective for data storage.

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic support; and a magnetic layer including a ferromagneticpowder, wherein an isoelectric point of a surface zeta potential of themagnetic layer after pressing the magnetic layer at a pressure of 70 atmis equal to or greater than 5.5.
 2. The magnetic recording mediumaccording to claim 1, wherein the isoelectric point is 5.5 to 7.0. 3.The magnetic recording medium according to claim 1, wherein the magneticlayer includes inorganic oxide-based particles.
 4. The magneticrecording medium according to claim 3, wherein the inorganic oxide-basedparticles are composite particles of an inorganic oxide and a polymer.5. The magnetic recording medium according to claim 1, wherein themagnetic layer includes a binding agent having an acidic group.
 6. Themagnetic recording medium according to claim 5, wherein the acidic groupis at least one kind of acidic group selected from the group consistingof sulfonic acid groups and salts thereof.
 7. The magnetic recordingmedium according to claim 1, further comprising: a non-magnetic layerincluding a non-magnetic powder between the non-magnetic support and themagnetic layer.
 8. The magnetic recording medium according to claim 1,further comprising: a back coating layer including a non-magnetic powderon a surface of the non-magnetic support opposite to a surface providedwith the magnetic layer.
 9. The magnetic recording medium according toclaim 1, wherein the magnetic recording medium is a magnetic tape.
 10. Amagnetic recording and reproducing device comprising: a magneticrecording medium; and a magnetic head, wherein the magnetic recordingmedium comprises: a non-magnetic support; and a magnetic layer includinga ferromagnetic powder, wherein an isoelectric point of a surface zetapotential of the magnetic layer after pressing the magnetic layer at apressure of 70 atm is equal to or greater than 5.5.
 11. The magneticrecording and reproducing device according to claim 10, wherein theisoelectric point is 5.5 to 7.0.
 12. The magnetic recording andreproducing device according to claim 10, wherein the magnetic layerincludes inorganic oxide-based particles.
 13. The magnetic recording andreproducing device according to claim 12, wherein the inorganicoxide-based particles are composite particles of an inorganic oxide anda polymer.
 14. The magnetic recording and reproducing device accordingto claim 10, wherein the magnetic layer includes a binding agent havingan acidic group.
 15. The magnetic recording and reproducing deviceaccording to claim 14, wherein the acidic group is at least one kind ofacidic group selected from the group consisting of sulfonic acid groupsand salts thereof.
 16. The magnetic recording and reproducing deviceaccording to claim 10, wherein the magnetic recording medium furthercomprises a non-magnetic layer including a non-magnetic powder betweenthe non-magnetic support and the magnetic layer.
 17. The magneticrecording and reproducing device according to claim 10, wherein themagnetic recording medium further comprises a back coating layerincluding a non-magnetic powder on a surface of the non-magnetic supportopposite to a surface provided with the magnetic layer.
 18. The magneticrecording and reproducing device according to claim 10, wherein themagnetic recording medium is a magnetic tape.